cmd_push: pass "flags" pointer to config callback

cmd_push: pass "flags" pointer to config callback

This will let us manipulate any transport flags which have matching
config options (there are none yet, but we will add one in
the next patch).

We could also just make "flags" a static file-scope global,
but the result is a little confusing. We end up passing it
along through do_push and push_with_options, each of which
further munge it. Having slightly-differing versions of the
flags variable available to those functions would probably
cause more confusion than it is worth. Let's just keep the
original local to cmd_push, and it can continue to pass it
through the call-stack.

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