submodule: absorb git dir instead of dying on deinit

submodule: absorb git dir instead of dying on deinit

Currently, running 'git submodule deinit' on repos where the
submodule's '.git' is a directory, aborts with a message that is not
exactly user friendly.

Let's change this to instead warn the user that the .git/ directory
has been absorbed into the superproject.
The rest of the deinit function can operate as it already does with
new-style submodules.

In one test, we used to require "git submodule deinit" to fail even
with the "--force" option when the submodule's .git/ directory is not
absorbed. Adjust it to expect the operation to pass.

Suggested-by: Atharva Raykar <raykar.ath@gmail.com>
Signed-off-by: Mugdha Pattnaik <mugdhapattnaik@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>