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>

cache.h

@@ -222,6 +222,7 @@ extern const char *get_git_work_tree(void);
 #define ALTERNATE_DB_ENVIRONMENT "GIT_ALTERNATE_OBJECT_DIRECTORIES"
 
 extern const char **get_pathspec(const char *prefix, const char **pathspec);
+extern void setup_work_tree(void);
 extern const char *setup_git_directory_gently(int *);
 extern const char *setup_git_directory(void);
 extern const char *prefix_path(const char *prefix, int len, const char *path);

git.c

@@ -249,14 +249,9 @@ static int run_command(struct cmd_struct *p, int argc, const char **argv)
 		prefix = setup_git_directory();
 	if (p->option & USE_PAGER)
 		setup_pager();
-	if (p->option & NEED_WORK_TREE) {
-		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("%s must be run in a work tree", p->cmd);
-	}
+	if (p->option & NEED_WORK_TREE)
+		setup_work_tree();
+
 	trace_argv_printf(argv, argc, "trace: built-in: git");
 
 	status = p->fn(argc, argv, prefix);

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.