Fix typo: existant->existent

refs.c had a error message "Trying to write ref with nonexistant object".
And no tests relied on the wrong spelling.
Also typo was present in some test scripts internals, these tests still pass.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/refs.c b/refs.c
index e3c0511..b10419a 100644
--- a/refs.c
+++ b/refs.c
@@ -1451,7 +1451,7 @@
 	}
 	o = parse_object(sha1);
 	if (!o) {
-		error("Trying to write ref %s with nonexistant object %s",
+		error("Trying to write ref %s with nonexistent object %s",
 			lock->ref_name, sha1_to_hex(sha1));
 		unlock_ref(lock);
 		return -1;