Refactor working tree setup

Create a setup_work_tree() that can be used from any command requiring
a working tree conditionally.

Signed-off-by: Mike Hommey <mh@glandium.org>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -206,6 +206,15 @@ static const char *set_work_tree(const char *dir)
 	return NULL;
 }
 
+void setup_work_tree(void) {
+	const char *work_tree = get_git_work_tree();
+	const char *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");
+}
+
 /*
  * We cannot decide in this function whether we are in the work tree or
  * not, since the config can only be read _after_ this function was called.