cmdutil: add origbackuppath helper

discovery: pass pushop to _nowarnheads Previously, we passed only enough information to _nowarnheads to perform the old calculation. However, extensions may want to use more information in this calculation. Now that pushop is extensible, extensions will often want access to the additional fields they have added to the pushop, so pass that through. Specifically, the remotenames extension will use this information to allow pushing new heads that are bookmarked without requiring --force.

exchange: pass pushop to discovery.checkheads Previously, we passed a bunch of parameters to discovery.checkheads, but all of the arguments can be fetched out of pushop, which may contain a lot more useful information for extensions now that pushop is extensible.