Set default MR title/description to first multi-line commit

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/300479

When creating a merge request, the behaviour of the prefilled title and
discription is different than the behaviour of squashed commit messages.

For squashed commit messages, the behaviour is described in the
[docs](https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html#overview)
as

> Taken from the first multi-line commit message in the merge.

However, for merge requests, the current behavior is to use the branch
name.  This commit sets the prefilled MR title/description to the first
multi-line commit in the MR, if applicable.  Otherwise falling back to
the previous behaviour.

app/models/merge_request.rb

@@ -1282,7 +1282,14 @@ class MergeRequest < ApplicationRecord
   # Returns the oldest multi-line commit message, or the MR title if none found
   def default_squash_commit_message
     strong_memoize(:default_squash_commit_message) do
-      recent_commits.without_merge_commits.reverse_each.find(&:description?)&.safe_message || title
+      first_multiline_commit&.safe_message || title
+    end
+  end
+
+  # Returns the oldest multi-line commit
+  def first_multiline_commit
+    strong_memoize(:first_multiline_commit) do
+      recent_commits.without_merge_commits.reverse_each.find(&:description?)
     end
   end
 

app/services/merge_requests/build_service.rb

@@ -58,6 +58,7 @@ module MergeRequests
              :compare_commits,
              :wip_title,
              :description,
+             :first_multiline_commit,
              :errors,
              to: :merge_request
 
@@ -196,7 +197,8 @@ module MergeRequests
     # interpreted as the user wants to close that issue on this project.
     #
     # For example:
-    # - Issue 112 exists, title: Emoji don't show up in commit title
+    # - Issue 112 exists
+    # - title: Emoji don't show up in commit title
     # - Source branch is: 112-fix-mep-mep
     #
     # Will lead to:
@@ -205,7 +207,7 @@ module MergeRequests
     #   more than one commit in the MR
     #
     def assign_title_and_description
-      assign_title_and_description_from_single_commit
+      assign_title_and_description_from_commits
       merge_request.title ||= title_from_issue if target_project.issues_enabled? || target_project.external_issue_tracker
       merge_request.title ||= source_branch.titleize.humanize
       merge_request.title = wip_title if compare_commits.empty?
@@ -240,12 +242,16 @@ module MergeRequests
       end
     end
 
-    def assign_title_and_description_from_single_commit
+    def assign_title_and_description_from_commits
       commits = compare_commits
 
-      return unless commits&.count == 1
-
+      if commits&.count == 1
         commit = commits.first
+      else
+        commit = first_multiline_commit
+        return unless commit
+      end
+
       merge_request.title ||= commit.title
       merge_request.description ||= commit.description.try(:strip)
     end

changelogs/unreleased/300479-change-prefilled-mr-title-description.yml

+---
+title: Prefill first multiline commit message for new MRs
+merge_request: 52984
+author: Max Coplan @vegerot
+type: changed

doc/user/project/merge_requests/creating_merge_requests.md

@@ -32,9 +32,12 @@ button and start a merge request from there.
 
 On the **New Merge Request** page, start by filling in the title
 and description for the merge request. If there are already
-commits on the branch, the title is prefilled with the first
-line of the first commit message, and the description is
-prefilled with any additional lines in the commit message.
+commits on the branch, the title is prefilled with either:
+- The first line of the first multi-line commit message
+  and the description is prefilled with any additional lines
+  in that commit message.
+- The branch name, if there are no multi-line commit messages.
+
 The title is the only field that is mandatory in all cases.
 
 From there, you can fill it with information (title, description,