New users tend to work on one branch at a time and push the result
out. The current and upstream modes of push is a more suitable default
mode than matching mode for these people, but neither is surprise-free
depending on how the project is set up. Introduce a "simple" mode that
is a subset of "upstream" but only works when the branch is named the same
between the remote and local repositories.

The plan is to make it the new default when push.default is not
configured.

By Matthieu Moy (5) and others
* mm/simple-push:
  push.default doc: explain simple after upstream
  push: document the future default change for push.default (matching -> simple)
  t5570: use explicit push refspec
  push: introduce new push.default mode "simple"
  t5528-push-default.sh: add helper functions
  Undocument deprecated alias 'push.default=tracking'
  Documentation: explain push.default option a bit more