builtin-blame: set up the work_tree before the first file access

We check in cmd_blame() if the specified path is there, but we
failed to set up the working tree before that.

While at it, make setup_work_tree() just return if it was run
before.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-blame.c

@@ -2295,6 +2295,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 			else if (i != argc - 1)
 				usage(blame_usage); /* garbage at end */
 
+			setup_work_tree();
 			if (!has_path_in_work_tree(path))
 				die("cannot stat path %s: %s",
 				    path, strerror(errno));