merge: fix incorrect merge message for ambiguous tag/branch

merge: fix incorrect merge message for ambiguous tag/branch

If we have both a tag and a branch named "foo", then calling
"git merge foo" will warn about the ambiguous ref, but merge
the tag.

When generating the commit message, though, we simply
checked whether "refs/heads/foo" existed, and if it did,
assumed it was a branch. This led to the statement "Merge
branch 'foo'" in the commit message, which is quite wrong.

Instead, we should use dwim_ref to find the actual ref used,
and describe it appropriately.

In addition to the test in t7608, we must also tweak the
expected output of t4202, which was accidentally triggering
this bug.

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