Support adding and removing assignees w/ push opts

There are already push options for many things already, like
to add and remove labels from a merge request. See:

https://docs.gitlab.com/ee/user/project/push_options.html

But there is no push option yet to add and remove assignees
from a merge request. Let's implement that.
Signed-off-by: Christian Couder <christian@gitlab.com>

app/services/merge_requests/push_options_handler_service.rb

@@ -129,7 +129,9 @@ module MergeRequests
         target_branch: push_options[:target],
         force_remove_source_branch: push_options[:remove_source_branch],
         label: push_options[:label],
-        unlabel: push_options[:unlabel]
+        unlabel: push_options[:unlabel],
+        assign: push_options[:assign],
+        unassign: push_options[:unassign]
       }
 
       params.compact!

changelogs/unreleased/assignee-push-option.yml

+---
+title: Support adding and removing assignees w/ push opts
+merge_request: 25904
+author:
+type: added

doc/user/project/push_options.md

@@ -68,6 +68,8 @@ time as pushing changes:
 | `merge_request.description="<description>"`  | Set the description of the merge request. Ex: `git push -o merge_request.description="The description I want"`. | [12.2](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/64320)          |
 | `merge_request.label="<label>"`              | Add labels to the merge request. If the label does not exist, it will be created. For example, for two labels: `git push -o merge_request.label="label1" -o merge_request.label="label2"`. | [12.3](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/31831) |
 | `merge_request.unlabel="<label>"`            | Remove labels from the merge request. For example, for two labels: `git push -o merge_request.unlabel="label1" -o merge_request.unlabel="label2"`. | [12.3](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/31831) |
+| `merge_request.assign="<user>"`              | Assign users to the merge request. For example, for two users: `git push -o merge_request.assign="user1" -o merge_request.assign="user2"`. | [12.9](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/XXXXX) |
+| `merge_request.unassign="<user>"`            | Remove assigned users from the merge request. For example, for two users: `git push -o merge_request.unassign="user1" -o merge_request.unassign="user2"`. | [12.9](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/XXXXX) |
 
 If you use a push option that requires text with spaces in it, you need to enclose it
 in quotes (`"`). You can omit the quotes if there are no spaces. Some examples:

lib/gitlab/push_options.rb

@@ -5,6 +5,7 @@ module Gitlab
     VALID_OPTIONS = HashWithIndifferentAccess.new({
       merge_request: {
         keys: [
+          :assign,
           :create,
           :description,
           :label,
@@ -12,6 +13,7 @@ module Gitlab
           :remove_source_branch,
           :target,
           :title,
+          :unassign,
           :unlabel
         ]
       },
@@ -23,7 +25,9 @@ module Gitlab
     MULTI_VALUE_OPTIONS = [
       %w[ci variable],
       %w[merge_request label],
-      %w[merge_request unlabel]
+      %w[merge_request unlabel],
+      %w[merge_request assign],
+      %w[merge_request unassign]
     ].freeze
 
     NAMESPACE_ALIASES = HashWithIndifferentAccess.new({