Merge branch 'fc/push-simple-updates-cleanup'

Some more code and doc clarification around "git push".

* fc/push-simple-updates-cleanup:
  push: don't get a full remote object
  push: only check same_remote when needed
  push: remove trivial function
  push: remove redundant check
  push: factor out the typical case
  push: get rid of all the setup_push_* functions
  push: trivial simplifications
  push: make setup_push_* return the dst
  push: only get the branch when needed
  push: factor out null branch check
  push: split switch cases
  push: return immediately in trivial switch case
  push: create new get_upstream_ref() helper