doc: git-push: clarify "what to push"

From user feedback: 6 users says they found the "what to push" paragraphs confusing, for many different reasons, including: * what does "..." in <refspec>... mean? * "consult XXX configuration" is hard to parse * it refers to the `git-config` man page even though the config information for `git push` is included in this man page under CONFIGURATION * the default ("push to a branch with the same name") is what they use 99% of the time, they would have expected it to appear earlier instead of at the very end * not understanding what the term "upstream" means in Git ("are branches tracked by some system besides their names?"") Also, the current explanation of `push.default=simple` ("the current branch is pushed to the corresponding upstream branch, but as a safety measure, the push is aborted if the upstream branch does not have the same name as the local one.") is not accurate: `push.default=simple` does not always require you to set a corresponding upstream branch. Address all of these by * using a numbered "in order of precedence" list * giving a more accurate explanation of how `push.default=simple` works * giving a little bit of context around "upstream branch": it's something that you may have to set explicitly * referring to the new UPSTREAM BRANCHES section The default behaviour is still discussed pretty late but it should be easier to skim now to get to the relevant information. In "`git push` may fail if...", I'm intentionally being vague about what exactly `git push` does, because (as discussed on the mailing list) the behaviour of `push.default=simple` is very confusing, perhaps broken, and certainly not worth trying to explain in an introductory context. `push.default.simple` sometimes requires you to set an upstream and sometimes doesn't and the exact conditions under which it does/doesn't are hard to describe. Signed-off-by: Julia Evans <julia@jvns.ca> Signed-off-by: Junio C Hamano <gitster@pobox.com>