get_sha1: detect buggy calls with multiple disambiguators

The get_sha1() family of functions takes a flags field, but
some of the flags are mutually exclusive. In particular, we
can only handle one disambiguating function, and the flags
quietly override each other. Let's instead detect these as
programming bugs.

Technically some of the flags are supersets of the others,
so treating COMMITTISH|TREEISH as just COMMITTISH is not
wrong, but it's a good sign the caller is confused. And
certainly asking for BLOB|TREE does not work.

We can do the check easily with some bit-twiddling, and as a
bonus, the bit-mask of disambiguators will come in handy in
a future patch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1203,6 +1203,11 @@ struct object_context {
 #define GET_SHA1_FOLLOW_SYMLINKS 0100
 #define GET_SHA1_ONLY_TO_DIE    04000
 
+#define GET_SHA1_DISAMBIGUATORS \
+	(GET_SHA1_COMMIT | GET_SHA1_COMMITTISH | \
+	GET_SHA1_TREE | GET_SHA1_TREEISH | \
+	GET_SHA1_BLOB)
+
 extern int get_sha1(const char *str, unsigned char *sha1);
 extern int get_sha1_commit(const char *str, unsigned char *sha1);
 extern int get_sha1_committish(const char *str, unsigned char *sha1);

sha1_name.c

@@ -327,6 +327,10 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 	prepare_alt_odb();
 
 	memset(&ds, 0, sizeof(ds));
+
+	if (HAS_MULTI_BITS(flags & GET_SHA1_DISAMBIGUATORS))
+		die("BUG: multiple get_short_sha1 disambiguator flags");
+
 	if (flags & GET_SHA1_COMMIT)
 		ds.fn = disambiguate_commit_only;
 	else if (flags & GET_SHA1_COMMITTISH)