builtin-blame: Fix blame -C -C with submodules.

When performing copy detection, git-blame tries to
read gitlinks as blobs, which causes it to die.

This patch adds a check to skip them.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

builtin-blame.c

@@ -1136,6 +1136,8 @@ static int find_copy_in_parent(struct scoreboard *sb,
 
 			if (!DIFF_FILE_VALID(p->one))
 				continue; /* does not exist in parent */
+			if (S_ISGITLINK(p->one->mode))
+				continue; /* ignore git links */
 			if (porigin && !strcmp(p->one->path, porigin->path))
 				/* find_move already dealt with this path */
 				continue;