submodule: prevent backslash expantion in submodule names

submodule: prevent backslash expantion in submodule names

When attempting to add a submodule with backslashes in its name 'git
submodule' fails in a funny way.  We can see that some of the
backslashes are expanded resulting in a bogus path:

git -C main submodule add ../sub\\with\\backslash
fatal: repository '/tmp/test/sub\witackslash' does not exist
fatal: clone of '/tmp/test/sub\witackslash' into submodule path

To solve this, convert calls to 'read' to 'read -r' in git-submodule.sh
in order to prevent backslash expantion in submodule names.

Reported-by: Joachim Durchholz <jo@durchholz.org>
Signed-off-by: Brandon Williams <bmwill@google.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>