pull: gather all bookmark-pulling code together Pulling bookmarks is done in two rounds. First we do a simple update, then we use the content of the `bookmark` argument to possibly overwrite some bookmark with their remote location. The second step was not done right after the first one for some obscure reason. We now perform them one after the other.

push: perform bookmark export in the push function This part is responsible for adding new bookmarks on the remote. Before that, it was done on its own in `commands.push`. The export is still not integrated with the rest of the push process, but at least it now dwells in the right function.

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.