sha1-file: add functions for hex empty tree and blob OIDs

Oftentimes, we'll want to refer to an empty tree or empty blob by its
hex name without having to call oid_to_hex or explicitly refer to
the_hash_algo.  Add helper functions that format these values into
static buffers and return them for easy use.

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

cache.h

@@ -1049,6 +1049,9 @@ static inline int is_empty_tree_oid(const struct object_id *oid)
 	return !oidcmp(oid, the_hash_algo->empty_tree);
 }
 
+const char *empty_tree_oid_hex(void);
+const char *empty_blob_oid_hex(void);
+
 /* set default permissions by passing mode arguments to open(2) */
 int git_mkstemps_mode(char *pattern, int suffix_len, int mode);
 int git_mkstemp_mode(char *pattern, int mode);

sha1_file.c

@@ -100,6 +100,18 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 	},
 };
 
+const char *empty_tree_oid_hex(void)
+{
+	static char buf[GIT_MAX_HEXSZ + 1];
+	return oid_to_hex_r(buf, the_hash_algo->empty_tree);
+}
+
+const char *empty_blob_oid_hex(void)
+{
+	static char buf[GIT_MAX_HEXSZ + 1];
+	return oid_to_hex_r(buf, the_hash_algo->empty_blob);
+}
+
 /*
  * This is meant to hold a *small* number of objects that you would
  * want read_sha1_file() to be able to return, but yet you do not want