setup_git_directory(): fix move to worktree toplevel directory

When setup_git_directory() returns successfully, it is supposed to move
current working directory to worktree toplevel directory.

However, the code recomputing prefix inside setup_git_directory() has
to move cwd back to original working directory, in order to get new
prefix.  After that, it should move cwd back to worktree toplevel
directory as expected.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

setup.c

@@ -581,6 +581,8 @@ const char *setup_git_directory(void)
 		if (retval && chdir(retval))
 			die ("Could not jump back into original cwd");
 		rel = get_relative_cwd(buffer, PATH_MAX, get_git_work_tree());
+		if (rel && *rel && chdir(get_git_work_tree()))
+			die ("Could not jump to working directory");
 		return rel && *rel ? strcat(rel, "/") : NULL;
 	}