Do not create a MergeRequestDiff record when source branch is deletedfix-merge-request-screen-deleted-source-branch

In order to have a valid MergeRequestDiff record when need head_commit_sha.
When a source branch is deleted head_commit_sha is nil.
This causes an exception in merge request "Changes" tab.

1 files changed, 12 insertions, 9 deletions

diff --git a/app/services/merge_requests/refresh_service.rb b/app/services/merge_requests/refresh_service.rb
index 22596b4014a..4a7e6930842 100644
--- a/app/services/merge_requests/refresh_service.rb
+++ b/app/services/merge_requests/refresh_service.rb
@@ -60,15 +60,7 @@ module MergeRequests
       merge_requests = filter_merge_requests(merge_requests)
 
       merge_requests.each do |merge_request|
-        if merge_request.source_branch == @branch_name || force_push?
-          merge_request.reload_diff
-        else
-          mr_commit_ids = merge_request.commits.map(&:id)
-          push_commit_ids = @commits.map(&:id)
-          matches = mr_commit_ids & push_commit_ids
-          merge_request.reload_diff if matches.any?
-        end
-
+        reload_diff(merge_request) unless branch_removed?
         merge_request.mark_as_unchecked
       end
     end
@@ -173,5 +165,16 @@ module MergeRequests
     def branch_removed?
       Gitlab::Git.blank_ref?(@newrev)
     end
+
+    def reload_diff(merge_request)
+      if merge_request.source_branch == @branch_name || force_push?
+        merge_request.reload_diff
+      else
+        mr_commit_ids = merge_request.commits.map(&:id)
+        push_commit_ids = @commits.map(&:id)
+        matches = mr_commit_ids & push_commit_ids
+        merge_request.reload_diff if matches.any?
+      end
+    end
   end
 end