Teach mv to 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>
Teach mv to 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.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Teach mv to update the path entry in .gitmodules for moved submodules

Currently using "git mv" on a submodule moves the submodule's work tree in
that of the superproject. But the submodule's path setting in .gitmodules
is left untouched, which is now inconsistent with the work tree and makes
git commands that rely on the proper path -> name mapping (like status and
diff) behave strangely.

Let "git mv" help here by not only moving the submodule's work tree but
also updating the "submodule.<submodule name>.path" setting from the
.gitmodules file and stage both. This doesn't happen when no .gitmodules
file is found and only issues a warning when it doesn't have a section for
this submodule. This is because the user might just use plain gitlinks
without the .gitmodules file or has already updated the path setting by
hand before issuing the "git mv" command (in which case the warning
reminds him that mv would have done that for him). Only when .gitmodules
is found and contains merge conflicts the mv command will fail and tell
the user to resolve the conflict before trying again.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>