sha1_name.c: introduce get_sha1_committish()

sha1_name.c: introduce get_sha1_committish()

Many callers know that the user meant to name a committish by
syntactical positions where the object name appears.  Calling this
function allows the machinery to disambiguate shorter-than-unique
abbreviated object names between committish and others.

Note that this does NOT error out when the named object is not a
committish. It is merely to give a hint to the disambiguation
machinery.

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