git-submodule.sh: make the "$cached" variable a boolean

git-submodule.sh: make the "$cached" variable a boolean

Remove the assignment of "$1" to the "$cached" variable. As seen in
the initial implementation in 70c7ac22de6 (Add git-submodule command,
2007-05-26) we only need to keep track of if we've seen the --cached
option, not save the "--cached" string for later use.

In 28f9af5d25e (git-submodule summary: code framework, 2008-03-11)
"$1" was assigned to it, but since there was no reason to do so let's
stop doing it. This trivial change will make it easier to reason about
an eventual change that'll remove the cmd_summary() function in favor
of dispatching to "git submodule--helper summary" directly.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>