merge: a random object may not necssarily be a commit

merge: a random object may not necssarily be a commit

The user could have said "git merge $(git rev-parse v1.0.0)"; we
shouldn't mark it as "Merge commit '15999998fb...'" as the merge
name, even though such an invocation might be crazy.

We could even read the "tag " header from the tag object and replace
the object name the user gave us, but let's not lose the information
by doing so, at least not yet.

Signed-off-by: Junio C Hamano <gitster@pobox.com>