Fix merge button dropdown caret horizontal alignment

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/37194

It looks like this regressed in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12601 when we
switched from `fa-caret-down` to `fa-chevron-down`

app/assets/stylesheets/pages/merge_requests.scss

@@ -291,6 +291,7 @@
 
     .dropdown-toggle {
       .fa {
+        margin-left: 0;
         color: inherit;
       }
     }

changelogs/unreleased/37194-fix-mr-widget-merge-button-dropdown-caret.yml

+---
+title: Fix Merge when pipeline succeeds button dropdown caret icon horizontal alignment
+merge_request:
+author:
+type: fixed