push: truly use "simple" as default, not "upstream"

The plan for the push.default transition had all along been to use the "simple" method rather than "upstream" as a default if the user did not specify their own push.default value. Commit 11037ee (push: switch default from "matching" to "simple", 2013-01-04) tried to implement that by moving PUSH_DEFAULT_UNSPECIFIED in our switch statement to fall-through to the PUSH_DEFAULT_SIMPLE case. When the commit that became 11037ee was originally written, that would have been enough. We would fall through to calling setup_push_upstream() with the "simple" parameter set to 1. However, it was delayed for a while until we were ready to make the transition in Git 2.0. And in the meantime, commit ed2b182 (push: change `simple` to accommodate triangular workflows, 2013-06-19) threw a monkey wrench into the works. That commit drops the "simple" parameter to setup_push_upstream, and instead checks whether the global "push_default" is PUSH_DEFAULT_SIMPLE. This is right when the user has explicitly configured push.default to simple, but wrong when we are a fall-through for the "unspecified" case. We never noticed because our push.default tests do not cover the case of the variable being totally unset; they only check the "simple" behavior itself. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>