git-submodule.sh: improve variables readability

When git-submodule.sh parses various options and switches, it sets some
variables to values; the variables in turn affect the options given to
git-submodule--helper.

Currently, variables which correspond to switches have boolean values
(for example, whenever "--force" is passed, force=1), while variables
which correspond to options which take arguments have string values that
sometimes contain the option name and sometimes only the option value.

Set all of the variables to strings which contain the option name (e.g.
force="--force" rather than force=1); this has a couple of advantages:
it improves consistency, readability and debuggability.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Roy Eldar <royeldar0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Roy Eldar