mv: move submodules together with their work trees

mv: move submodules together with their work trees

Currently the attempt to use "git mv" on a submodule errors out with:

  fatal: source directory is empty, source=<src>, destination=<dest>

The reason is that mv searches for the submodule with a trailing slash in
the index, which it doesn't find (because it is stored without a trailing
slash). As it doesn't find any index entries inside the submodule it
claims the directory would be empty even though it isn't.

Fix that by searching for the name without a trailing slash and continue
if it is a submodule. Then rename() will move the submodule work tree just
like it moves a file.

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