bundlerepo: let bundle repo look in the _mancache When looking up a base revision, we were ignoring the contents that were already available in the manifest's _mancache. This patch allows us to use that data instead of reading from the revlog. This is useful in our pushrebase extension (which allows rebasing on the server side during a push) because it allows us to prefetch the bundle base manifest before aquiring the repo lock (1 second saving), which means doing less work inside the lock, which means a 20% higher commit rate.