make is_empty_blob_sha1 available everywhere

The read-cache implementation defines this static function,
but it is a generally useful concept in git. Let's give
the empty blob the same treatment as the empty tree,
providing both hex and binary forms of the sha1.

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

cache.h

@@ -708,6 +708,19 @@ static inline void hashclr(unsigned char *hash)
 #define EMPTY_TREE_SHA1_BIN \
 	 ((const unsigned char *) EMPTY_TREE_SHA1_BIN_LITERAL)
 
+#define EMPTY_BLOB_SHA1_HEX \
+	"e69de29bb2d1d6434b8b29ae775ad8c2e48c5391"
+#define EMPTY_BLOB_SHA1_BIN_LITERAL \
+	"\xe6\x9d\xe2\x9b\xb2\xd1\xd6\x43\x4b\x8b" \
+	"\x29\xae\x77\x5a\xd8\xc2\xe4\x8c\x53\x91"
+#define EMPTY_BLOB_SHA1_BIN \
+	((const unsigned char *) EMPTY_BLOB_SHA1_BIN_LITERAL)
+
+static inline int is_empty_blob_sha1(const unsigned char *sha1)
+{
+	return !hashcmp(sha1, EMPTY_BLOB_SHA1_BIN);
+}
+
 int git_mkstemp(char *path, size_t n, const char *template);
 
 int git_mkstemps(char *path, size_t n, const char *template, int suffix_len);

read-cache.c

@@ -157,16 +157,6 @@ static int ce_modified_check_fs(struct cache_entry *ce, struct stat *st)
 	return 0;
 }
 
-static int is_empty_blob_sha1(const unsigned char *sha1)
-{
-	static const unsigned char empty_blob_sha1[20] = {
-		0xe6,0x9d,0xe2,0x9b,0xb2,0xd1,0xd6,0x43,0x4b,0x8b,
-		0x29,0xae,0x77,0x5a,0xd8,0xc2,0xe4,0x8c,0x53,0x91
-	};
-
-	return !hashcmp(sha1, empty_blob_sha1);
-}
-
 static int ce_match_stat_basic(struct cache_entry *ce, struct stat *st)
 {
 	unsigned int changed = 0;