mv: move submodules using a gitfile

mv: move submodules using a gitfile

When moving a submodule which uses a gitfile to point to the git directory
stored in .git/modules/<name> of the superproject two changes must be made
to make the submodule work: the .git file and the core.worktree setting
must be adjusted to point from work tree to git directory and back.

Achieve that by remembering which submodule uses a gitfile by storing the
result of read_gitfile() of each submodule. If that is not NULL the new
function connect_work_tree_and_git_dir() is called after renaming the
submodule's work tree which updates the two settings to the new values.

Extend the man page to inform the user about that feature (and while at it
change the description to not talk about a script anymore, as mv is a
builtin for quite some time now).

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>