Squash commits fails for branches named HEAD

With certain older branches, when we try and use the squash commits feature, we get something like following error message:

 "Something went wrong during merge: Failed to parse revision specifier - Invalid pattern '79e54e1e4abb756d229c86e2a50172a07b16d161 warning: refname 'HEAD' is ambiguous.'"

Note: Here the branch name is head

ZD: https://gitlab.zendesk.com/agent/tickets/67565

cc// @smcgivern