Fix squashing when a ref is named HEAD

This would print an ambiguous refs warning, so silence that with
--quiet. HEAD in this context will refer to the 'current commit', not
the ref with that name.

app/services/merge_requests/squash_service.rb

@@ -50,8 +50,12 @@ module MergeRequests
         'commit squashed changes'
       )
 
+      # May print a warning for ambiguous refs, but we can ignore that with
+      # `--quiet` and just take the SHA, if present. HEAD here always refers to
+      # the current HEAD commit, even if there is another ref called HEAD.
+      #
       squash_sha = run_git_command(
-        %w(rev-parse HEAD),
+        %w(rev-parse --quiet --verify HEAD),
         tree_path,
         git_env,
         'get SHA of squashed commit'

changelogs/unreleased-ee/1861-squash-commits-fails-with-older-branches.yml

+---
+title: Fix squashing MRs when the repository contains a ref named HEAD
+merge_request:
+author: