set_git_dir: handle feeding gitdir to itself

Ideally we'd free the existing gitdir field before assigning the new one, to avoid a memory leak. But we can't do so safely because some callers do the equivalent of: set_git_dir(get_git_dir()); We can detect that case as a noop, but there are even more complicated cases like: set_git_dir(remove_leading_path(worktree, get_git_dir()); where we really do need to do some work, but the original string must remain valid. Rather than put the burden on callers to make a copy of the string (only to free it later, since we'll make a copy of it ourselves), let's solve the problem inside set_git_dir(). We can make a copy of the pointer for the old gitdir, and then avoid freeing it until after we've made our new copy. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>