Add documentation for /rebase quick action

This documents the use of /rebase for merge requests.

app/services/merge_requests/update_service.rb

@@ -127,12 +127,12 @@ module MergeRequests
     def handle_quick_actions(merge_request)
       super
 
-      # Ensure this parameter gets used as an attribute
+      # Ensure this parameter does not get used as an attribute
       rebase = params.delete(:rebase)
 
       if rebase
         rebase_from_quick_action(merge_request)
-        # Ignore /merge if /rebase is used to avoid an unexpected race
+        # Ignore "/merge" if "/rebase" is used to avoid an unexpected race
         params.delete(:merge)
       end
 

changelogs/unreleased/sh-quick-action-rebase.yml

+---
+title: Add a quick action for /rebase
+merge_request: 49800
+author:
+type: added

doc/user/project/quick_actions.md

@@ -56,6 +56,7 @@ The following quick actions are applicable to descriptions, discussions and thre
 | `/promote`                            | ✓     |               |      | Promote issue to epic. **(PREMIUM)**                                                                                            |
 | `/publish`                            | ✓     |               |      | Publish issue to an associated [Status Page](../../operations/incident_management/status_page.md) ([Introduced in GitLab 13.0](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30906)) **(ULTIMATE)** |
 | `/reassign @user1 @user2`             | ✓     | ✓             |      | Replace current assignees with those specified. **(STARTER)**                                                                                         |
+| `/rebase`                             |       | ✓             |      | Rebase source branch. This will schedule a background task that attempt to rebase the changes in the source branch on the latest commit of the target branch. If `/rebase` is used, `/merge` will be ignored to avoid a race condition where the source branch is merged or deleted before it is rebased. |
 | `/relabel ~label1 ~label2`            | ✓     | ✓             | ✓    | Replace current labels with those specified.                                                                        |
 | `/relate #issue1 #issue2`             | ✓     |               |      | Mark issues as related. **(STARTER)**                                                                                            |
 | `/remove_child_epic <epic>`           |       |               | ✓    | Remove child epic from `<epic>`. The `<epic>` value should be in the format of `&epic`, `group&epic`, or a URL to an epic ([introduced in GitLab 12.0](https://gitlab.com/gitlab-org/gitlab/-/issues/7330)). **(ULTIMATE)** |

lib/gitlab/quick_actions/merge_request_actions.rb

@@ -56,7 +56,7 @@ module Gitlab
           access_check.can_push_to_branch?(merge_request.source_branch)
         end
         command :rebase do
-          # This will be used to avoid simultaneous /merge and /rebase actions
+          # This will be used to avoid simultaneous "/merge" and "/rebase" actions
           @updates[:rebase] = true
 
           branch = quick_action_target.source_branch