submodule: use capture_command

submodule: use capture_command

In is_submodule_commit_present, we call run_command followed
by a pipe read, which is prone to deadlock. It is unlikely
to happen in this case, as rev-list should never produce
more than a single line of output, but it does not hurt to
avoid an anti-pattern (and using the helper simplifies the
setup and cleanup).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>