help_unknown_ref(): check for refname ambiguity

help_unknown_ref(): check for refname ambiguity

When the user asks to merge "foo" and we suggest "origin/foo" instead,
we do so by simply chopping off "refs/remotes/" from the front of the
suggested ref. This is usually fine, but it's possible that the
resulting name is ambiguous (e.g., you have "refs/heads/origin/foo",
too).

Let's use shorten_unambiguous_ref() to do this the right way, which
should usually yield the same "origin/foo", but "remotes/origin/foo" if
necessary.

Note that in this situation there may be other options (e.g., we could
suggest "heads/origin/foo" as well). I'll leave that up for debate; the
focus here is just to avoid giving advice that does not actually do what
we expect.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>