sha1_file.c: introduce a null_oid constant

null_oid is the struct object_id equivalent to null_sha1.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -813,6 +813,7 @@ extern const char *find_unique_abbrev(const unsigned char *sha1, int len);
 extern int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len);
 
 extern const unsigned char null_sha1[GIT_SHA1_RAWSZ];
+extern const struct object_id null_oid;
 
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {

sha1_file.c

@@ -36,6 +36,7 @@
 static inline uintmax_t sz_fmt(size_t s) { return s; }
 
 const unsigned char null_sha1[20];
+const struct object_id null_oid;
 
 /*
  * This is meant to hold a *small* number of objects that you would