mergestate: add a cached property accessor for the local context This is going to be useful in an upcoming patch. We make this a public accessor because this is also going to be useful for custom merge drivers.

mergestate: raise exception if otherctx is accessed but _other isn't set We don't want to inadvertently return the workingctx (self._repo[None]).

pager: improve help for --pager= try to clarify how to enable/disable the pager