Fix Repository#has_ambiguous_refs? regex
What does this MR do?
This regex intended to check if any refs that do not contain a slash
were a prefix of a ref containing the slash, plus the slash. That is:
abc is a prefix of abc/def, but not of abcd/ef.
However, the regex grouping was incorrect, and we were creating a regex
like %r{^abc|def/}, which doesn't contain parentheses, and so would
actually match plain abc (it wouldn't match def/).
This method was originally added in !38484 (merged). I also had a different fix in !39707 (merged), because apparently I cannot get this right