sha1_name.c: many short names can only be committish

sha1_name.c: many short names can only be committish

We know that the token "$name" that appear in "$name^{commit}",
"$name^4", "$name~4" etc. can only name a committish (either a
commit or a tag that peels to a commit).  Teach get_short_sha1() to
take advantage of that knowledge when disambiguating an abbreviated
SHA-1 given as an object name.

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