[PATCH] allow merging any committish

[PATCH] allow merging any committish

Although "git-merge" is advertised as the end-user level command
(instead of being a "git-pull" backend), it was not prepared to
take tag objects that point at commits and barfed when fed one.
Sanitize the input while we validate them, for which we already
have a loop.

Signed-off-by: Junio C Hamano <junkio@cox.net>