gitignore: git-pack-refs is a generated file.

Signed-off-by: Junio C Hamano <junkio@cox.net>

.gitignore

@@ -73,6 +73,7 @@ git-name-rev
 git-mv
 git-pack-redundant
 git-pack-objects
+git-pack-refs
 git-parse-remote
 git-patch-id
 git-peek-remote