Commit with `Fixes #xxxx` won't be referenced in issue on non-default branch

There seems to be a tiny bug with the way Fixes #xxxx and #xxxx in commit messages get referenced in the issue log.

• Current behavior: when I write Fixes #xxxx in a commit message, no "reference" appears in the issue log. But if I write #xxxx (without the Fixes keyword), a "reference" to that commit appears in the issue log, e.g. Mentioned in commit 34a80c5.
• Expected behavior: both Fixes #xxxx and #xxxx (without the prefix Fixes) should trigger a "reference" in the issue log.

I found this problem on Gitlab CE 7.13.2.