sha1_name.c: teach get_short_sha1() a commit-only option

sha1_name.c: teach get_short_sha1() a commit-only option

When the caller knows that the parameter is meant to name a commit,
e.g. "56789a" in describe name "v1.2.3-4-g56789a", pass that as a
hint so that lower level can use it to disambiguate objects when
there is only one commit whose name begins with 56789a even if there
are objects of other types whose names share the same prefix.

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