Merge branch '14564-mr-automatic-title' into 'master'

Auto-set title for branches created from issues

This sets the title for a new MR to 'Resolves "$issue-title"' when:
- The source branch for the MR begins with a value iid.
- The MR has more than one commit in its diff (if there's one commit, keep using the commit's first line).
- The iid does not point to a confidential issue.

Single commit:

![A single commit uses the commit title](/uploads/cd34f59cd67f095c3034fae07950f8b5/image.png)

Multiple commits:

![Multiple commits use the issue title](/uploads/a322c406ddd56913c5aebd88d16e5a5e/image.png)

Confidential issue:

![A confidential issue uses the branch name](/uploads/7ae9b79de5f6101ced46802f3c3a6e71/image.png)

cc @DouweM @zj 

Closes #14564

See merge request !3966

1 files changed, 30 insertions, 6 deletions

diff --git a/app/services/merge_requests/build_service.rb b/app/services/merge_requests/build_service.rb
index 3544752d47a..cd4230aa5e4 100644
--- a/app/services/merge_requests/build_service.rb
+++ b/app/services/merge_requests/build_service.rb
@@ -41,21 +41,45 @@ module MergeRequests
         merge_request.can_be_created = false
       end
 
+      set_title_and_description(merge_request)
+    end
+
+    private
+
+    # When your branch name starts with an iid followed by a dash this pattern will be
+    # 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
+    # - Source branch is: 112-fix-mep-mep
+    #
+    # Will lead to:
+    # - Appending `Closes #112` to the description
+    # - Setting the title as 'Resolves "Emoji don't show up in commit title"' if there is
+    #   more than one commit in the MR
+    #
+    def set_title_and_description(merge_request)
+      if match = merge_request.source_branch.match(/\A(\d+)-/)
+        iid = match[1]
+      end
+
       commits = merge_request.compare_commits
       if commits && commits.count == 1
         commit = commits.first
         merge_request.title       = commit.title
         merge_request.description ||= commit.description.try(:strip)
+      elsif iid && (issue = merge_request.target_project.get_issue(iid)) && !issue.try(:confidential?)
+        case issue
+        when Issue
+          merge_request.title = "Resolve \"#{issue.title}\""
+        when ExternalIssue
+          merge_request.title = "Resolve #{issue.title}"
+        end
       else
         merge_request.title = merge_request.source_branch.titleize.humanize
       end
 
-      # When your branch name starts with an iid followed by a dash this pattern will
-      # be interpreted as the use wants to close that issue on this project
-      # Pattern example: 112-fix-mep-mep
-      # Will lead to appending `Closes #112` to the description
-      if match = merge_request.source_branch.match(/\A(\d+)-/)
-        iid = match[1]
+      if iid
         closes_issue = "Closes ##{iid}"
 
         if merge_request.description.present?