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>