push: use `exchange.push` in `commands.push`
To gain access to all results from the push, we need to have access to the
`pushoperation` object. We call `exchange.push` to do so.
It is impossible to just change the `localrepo.push` signature because the
change may be too subtle to be caught by external extension wrapping
`localrepo.push`.
This mean we'll have to kill `localrepo.push` because just using
`exchange.push` in `commands.py` would silently disable all wrapping around
`localrepo.push` by third-party extensions. So we'll remove it in a later
changeset to get such extensions to fail noisily.