Merge branch 'z-index-fix-for-diff-file-dropdown' into 'master'

z-index fixed for diff file dropdown

See merge request gitlab-org/gitlab-ce!30657

app/assets/stylesheets/pages/diff.scss

@@ -14,7 +14,7 @@
       position: -webkit-sticky;
       position: sticky;
       top: $mr-file-header-top;
-      z-index: 220;
+      z-index: 120;
 
       &::before {
         content: '';

changelogs/unreleased/z-index-fix-for-diff-file-dropdown.yml

+---
+title: I fixed z index bug in diff page
+merge_request: 30657
+author: Faruk Can
+type: fixed