sha1_name.c: allow get_short_sha1() to take other flags

Instead of a separate "int quietly" argument, make it take "unsigned
flags" so that we can pass other options to it.

The bit assignment of this flag word is exposed in cache.h because
the mechanism will be exposed to callers of the higher layer in
later commits in this series.

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

cache.h

@@ -811,6 +811,8 @@ struct object_context {
 	unsigned mode;
 };
 
+#define GET_SHA1_QUIETLY 01
+
 extern int get_sha1(const char *str, unsigned char *sha1);
 extern void maybe_die_on_misspelt_object_name(const char *name, const char *prefix);
 extern int get_sha1_with_context(const char *str, unsigned char *sha1, struct object_context *orc);