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>

cache.h

@@ -812,6 +812,7 @@ struct object_context {
 };
 
 #define GET_SHA1_QUIETLY 01
+#define GET_SHA1_COMMIT 02
 
 extern int get_sha1(const char *str, unsigned char *sha1);
 extern void maybe_die_on_misspelt_object_name(const char *name, const char *prefix);