bundlerepo: unify common code into a new getremotechanges The pattern where we fetch incoming remote changes and return them as a local bundlerepo seems common. It's nicer to have this code unified.

revset id(): fix error text to say "id wants..." instead of "rev wants..."

keyword: only use expensive fctx.cmp when needed Restrict expensive cmp to cases when: - comparing against working directory and - encode filters active or - path is configured for keyword expansion

context: narrow down filter special case in filectx.cmp() cmp via filelog when encode filters are present, but only when actually comparing to the working directory.