define empty tree sha1 as a macro

This can potentially be used in a few places, so let's make
it available to all parts of the code.

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

cache.h

@@ -528,6 +528,12 @@ static inline void hashclr(unsigned char *hash)
 }
 extern int is_empty_blob_sha1(const unsigned char *sha1);
 
+#define EMPTY_TREE_SHA1_HEX \
+	"4b825dc642cb6eb9a060e54bf8d69288fbee4904"
+#define EMPTY_TREE_SHA1_BIN \
+	 "\x4b\x82\x5d\xc6\x42\xcb\x6e\xb9\xa0\x60" \
+	 "\xe5\x4b\xf8\xd6\x92\x88\xfb\xee\x49\x04"
+
 int git_mkstemp(char *path, size_t n, const char *template);
 
 /*

sha1_file.c

@@ -1996,9 +1996,7 @@ static struct cached_object {
 static int cached_object_nr, cached_object_alloc;
 
 static struct cached_object empty_tree = {
-	/* empty tree sha1: 4b825dc642cb6eb9a060e54bf8d69288fbee4904 */
-	"\x4b\x82\x5d\xc6\x42\xcb\x6e\xb9\xa0\x60"
-	"\xe5\x4b\xf8\xd6\x92\x88\xfb\xee\x49\x04",
+	EMPTY_TREE_SHA1_BIN,
 	OBJ_TREE,
 	"",
 	0