repair: refactor broken linkrev collection This refactors broken linkrev collection such that manifest collection is in a separate function. This allows extensions to replace the manifest collection with a non-revlog oriented version. I considered moving the collect changes function onto the manifestlog itself, so it would be behind the abstraction, but since the store we're building doesn't even have the concept of strip, embeding that concept in the manifestlog api seemed odd. Differential Revision: https://phab.mercurial-scm.org/D291