sha1_file: introduce has_object_file helper.

Add has_object_file, which is a wrapper around has_sha1_file, but for
struct object_id.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>

cache.h

@@ -1030,6 +1030,9 @@ static inline int has_sha1_file(const unsigned char *sha1)
 	return has_sha1_file_with_flags(sha1, 0);
 }
 
+/* Same as the above, except for struct object_id. */
+extern int has_object_file(const struct object_id *oid);
+
 /*
  * Return true iff an alternate object database has a loose object
  * with the specified name.  This function does not respect replace