Add merge commit template field in project settings and docs

This change allows editing project's merge_commit_template
in project settings. Additionally it adds documentation
on merge commit template feature.

app/views/projects/_merge_request_merge_commit_template.html.haml

+- form = local_assigns.fetch(:form)
+
+.form-group
+  %b= s_('ProjectSettings|Merge commit message template')
+  %p.text-secondary
+    - configure_the_merge_commit_message_help_link_url = help_page_path('user/project/merge_requests/commit_templates.md', anchor: 'merge-commit-message-template')
+    - configure_the_merge_commit_message_help_link_start = '<a href="%{url}" target="_blank" rel="noopener noreferrer">'.html_safe % { url: configure_the_merge_commit_message_help_link_url }
+    = s_('ProjectSettings|The commit message used when merging, if the merge method creates a merge commit. %{link_start}Learn more about syntax and variables.%{link_end}').html_safe % { link_start: configure_the_merge_commit_message_help_link_start, link_end: '</a>'.html_safe }
+  .mb-2
+    - default_merge_commit_template = "Merge branch '%{source_branch}' into '%{target_branch}'\n\n%{title}\n\n%{issues}\n\nSee merge request %{reference}"
+    = form.text_area :merge_commit_template, class: 'form-control gl-form-input', rows: 8, maxlength: 500, placeholder: default_merge_commit_template
+    %p.form-text.text-muted
+      = s_('ProjectSettings|Maximum 500 characters.')
+      = s_('ProjectSettings|Supported variables:')
+      - Gitlab::MergeRequests::MergeCommitMessage::PLACEHOLDERS.keys.each do |placeholder|
+        %code
+          = "%{#{placeholder}}".html_safe

app/views/projects/_merge_request_settings.html.haml

@@ -10,5 +10,7 @@
 
 = render 'projects/merge_request_merge_suggestions_settings', project: @project, form: form
 
+= render 'projects/merge_request_merge_commit_template', project: @project, form: form
+
 - if @project.forked?
   = render 'projects/merge_request_target_project_settings', project: @project, form: form

doc/user/project/merge_requests/commit_templates.md

+---
+stage: Create
+group: Code Review
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+type: reference, howto
+---
+
+# Commit message templates **(FREE)**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/20263) in GitLab 14.5.
+
+## Merge commit message template
+
+As a project maintainer, you're able to configure merge commit message template. It will be used during merge to
+create commit message. Template uses similar syntax to 
+[review suggestions](reviews/suggestions.md#configure-the-commit-message-for-applied-suggestions).
+
+Default merge commit message can be recreated using following template:
+
+```plaintext
+Merge branch '%{source_branch}' into '%{target_branch}'
+
+%{title}
+
+%{issues}
+
+See merge request %{reference}
+```
+
+This commit message can be customized to follow any guidelines you might have.
+To do so, expand the **Merge requests** tab within your project's **General**
+settings and change the **Merge commit message template** text:
+
+![Custom commit message for applied suggestions](img/merge_commit_message_template_v14_5.png)
+
+You can use static text and following variables:
+
+| Variable           | Description                                                                                                                                                                                                                               | Output example                                  |
+|--------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------|
+| `%{source_branch}` | The name of the branch that is being merged.                                                                                                                                                                                              | `my-feature-branch`                             |
+| `%{target_branch}` | The name of the branch that the changes are applied to.                                                                                                                                                                                   | `master`                                        |
+| `%{title}`         | Title of the merge request.                                                                                                                                                                                                               | Fix stuff                                       |
+| `%{issues}`        | String with phrase "Closes <issue numbers>" with all issues mentioned in the MR description matching [issue closing patterns](../issues/managing_issues.md#closing-issues-automatically). It will be empty when no issues were mentioned. | `Closes #465, #190 and #400`                    |
+| `%{description}`   | Description of the merge request.                                                                                                                                                                                                         | Merge request description.<br>Can be multiline. |
+| `%{reference}`     | Reference to the merge request.                                                                                                                                                                                                           | group-name/project-name!72359                                          |
+
+NOTE:
+Empty variables that are the only word in a line will be removed along with all newline characters preceding it.
+
+Merge commit template field has a limit of 500 characters. This limit only applies to the template
+itself.

doc/user/project/settings/index.md

@@ -310,6 +310,7 @@ Set up your project's merge request settings:
 - Enable [require an associated issue from Jira](../../../integration/jira/issues.md#require-associated-jira-issue-for-merge-requests-to-be-merged).
 - Enable [`delete source branch after merge` option by default](../merge_requests/getting_started.md#deleting-the-source-branch).
 - Configure [suggested changes commit messages](../merge_requests/reviews/suggestions.md#configure-the-commit-message-for-applied-suggestions).
+- Configure [merge commit message template](../merge_requests/commit_templates.md).
 - Configure [the default target project](../merge_requests/creating_merge_requests.md#set-the-default-target-project) for merge requests coming from forks.
 
 ### Service Desk

ee/app/views/projects/_merge_request_settings.html.haml

@@ -13,6 +13,8 @@
 
 = render 'projects/merge_request_merge_suggestions_settings', project: @project, form: form
 
+= render_ce 'projects/merge_request_merge_commit_template', project: @project, form: form
+
 - if @project.forked?
   = render_ce 'projects/merge_request_target_project_settings', project: @project, form: form
 

locale/gitlab.pot

@@ -27144,12 +27144,18 @@ msgstr ""
 msgid "ProjectSettings|Manages large files such as audio, video, and graphics files."
 msgstr ""
 
+msgid "ProjectSettings|Maximum 500 characters."
+msgstr ""
+
 msgid "ProjectSettings|Merge checks"
 msgstr ""
 
 msgid "ProjectSettings|Merge commit"
 msgstr ""
 
+msgid "ProjectSettings|Merge commit message template"
+msgstr ""
+
 msgid "ProjectSettings|Merge commit with semi-linear history"
 msgstr ""
 
@@ -27267,6 +27273,9 @@ msgstr ""
 msgid "ProjectSettings|The commit message used when applying merge request suggestions. %{link_start}Learn more about suggestions.%{link_end}"
 msgstr ""
 
+msgid "ProjectSettings|The commit message used when merging, if the merge method creates a merge commit. %{link_start}Learn more about syntax and variables.%{link_end}"
+msgstr ""
+
 msgid "ProjectSettings|The default target project for merge requests created in this fork project."
 msgstr ""
 

spec/views/projects/edit.html.haml_spec.rb

@@ -57,6 +57,41 @@ RSpec.describe 'projects/edit' do
     end
   end
 
+  context 'merge commit template' do
+    it 'displays all possible variables' do
+      render
+
+      expect(rendered).to have_content('%{source_branch}')
+      expect(rendered).to have_content('%{target_branch}')
+      expect(rendered).to have_content('%{title}')
+      expect(rendered).to have_content('%{issues}')
+      expect(rendered).to have_content('%{description}')
+      expect(rendered).to have_content('%{reference}')
+    end
+
+    it 'displays a placeholder if none is set' do
+      render
+
+      expect(rendered).to have_field('project[merge_commit_template]', placeholder: <<~MSG.rstrip)
+        Merge branch '%{source_branch}' into '%{target_branch}'
+
+        %{title}
+
+        %{issues}
+
+        See merge request %{reference}
+      MSG
+    end
+
+    it 'displays the user entered value' do
+      project.update!(merge_commit_template: '%{title}')
+
+      render
+
+      expect(rendered).to have_field('project[merge_commit_template]', with: '%{title}')
+    end
+  end
+
   context 'forking' do
     before do
       assign(:project, project)