push: add `pushoperation.bkresult` This attribute will be used to carry the result of the bookmark push (when the `push` function will effectively be doing any bookmarks pushes)

push: pass list of bookmark to `exchange.push` Currently stored but not unused. This parameter will control bookmarks explicitly pushed (added to the server if missing).

exchange: import bookmarks as bookmod Using the original names makes it difficult to use `bookmarks` as a variable name.

push: sanitize handling of bookmark push return value Mixing return and assignment does not make sense, let's unify this.

locarepo: remove the `push` method (API) All the logic of this function is in the exchange.push function for some time. We just stop calling `localrepo.push` in `command.push` to have access to more information. Leaving `localrepo.push` in place will let third-party extensions wrap it but it would never be called by `hg push` making the wrapping useless. Therefore, the method is removed so that third-party code fail noisily and get properly upgraded.

push: `exchange.push` instead of `localrepo.push` The latter is going away.

clone: use `exchange.push` instead of `localrepo.push` The latter is going away.

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.