builtin/receive-pack: convert one use of EMPTY_TREE_SHA1_HEX

Convert one use of EMPTY_TREE_SHA1_HEX to use empty_tree_oid_hex to
avoid a dependency on a given hash algorithm.

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

builtin/receive-pack.c

@@ -968,7 +968,7 @@ static const char *push_to_deploy(unsigned char *sha1,
 		return "Working directory has unstaged changes";
 
 	/* diff-index with either HEAD or an empty tree */
-	diff_index[4] = head_has_history() ? "HEAD" : EMPTY_TREE_SHA1_HEX;
+	diff_index[4] = head_has_history() ? "HEAD" : empty_tree_oid_hex();
 
 	child_process_init(&child);
 	child.argv = diff_index;