Merge branch 'mm/push-default-switch-warning'

Merge branch 'mm/push-default-switch-warning'

In the next major release, we will switch "git push [$there]" that
does not say what to push from the traditional "matching" to the
updated "simple" semantics, that pushes the current branch to the
branch with the same name only when the current branch is set to
integrate with that remote branch (all other cases will error out).

* mm/push-default-switch-warning:
  push: start warning upcoming default change for push.default