FR: `revsets.push`

Is your feature request related to a problem? Please describe. basically, #2434. except i have a custom revset i'd like to use instead of whatever the default may end up being

Describe the solution you'd like a new config option akin to revsets.log: revsets.push

Describe alternatives you've considered since push is backend-specific behavior, it might be better to make the new config key git.push-revset instead. (NB would be nice to do some config squashing, ie git.push.revset, but that's a larger change)