rename dirlink to gitlink.

Unify naming of plumbing dirlink/gitlink concept:

git ls-files -z '*.[ch]' |
xargs -0 perl -pi -e 's/dirlink/gitlink/g;' -e 's/DIRLNK/GITLINK/g;'

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/diff.c b/diff.c
index b23e190..0e26049 100644
--- a/diff.c
+++ b/diff.c
@@ -1465,7 +1465,7 @@
 	if (size_only && 0 < s->size)
 		return 0;
 
-	if (S_ISDIRLNK(s->mode))
+	if (S_ISGITLINK(s->mode))
 		return diff_populate_gitlink(s, size_only);
 
 	if (!s->sha1_valid ||