status: respect "-b" for porcelain format

status: respect "-b" for porcelain format

There is no reason not to, as the user has to explicitly ask
for it, so we are not breaking compatibility by doing so. We
can do this simply by moving the "show_branch" flag into
the wt_status struct. As a bonus, this saves us from passing
it explicitly, simplifying the code.

Signed-off-by: Jeff King <peff@peff.net>