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));

setup.c

@@ -208,12 +208,18 @@ static const char *set_work_tree(const char *dir)
 
 void setup_work_tree(void)
 {
-	const char *work_tree = get_git_work_tree();
-	const char *git_dir = get_git_dir();
+	const char *work_tree, *git_dir;
+	static int initialized = 0;
+
+	if (initialized)
+		return;
+	work_tree = get_git_work_tree();
+	git_dir = get_git_dir();
 	if (!is_absolute_path(git_dir))
 		set_git_dir(make_absolute_path(git_dir));
 	if (!work_tree || chdir(work_tree))
 		die("This operation must be run in a work tree");
+	initialized = 1;
 }
 
 /*