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.

push: `exchange.push` now returns the `pushoperation` object Returning the pushop object gives access to more information (upcoming bookmark push result for example). `localrepo.push` currently extracts the `cgresult` for callers.

push: rename `pushop.ret` to `pushop.cgresult` We are about to introduce more results-related attributes on pushop (for bookmarks) so we need a more distinctive name. We now use `cgresult` as `pulloperation` does.

obsolete: use version constants in the format mapping