memctx: fix memctx manifest file hashes When memctx is asked for a manifest, it constructs one by merging the p1 manifest, and the changes that are on top. For the changes on top, it was previously using p1.node() as the file entries parent, which actually returns the commit node that the p1 linkrev points at! Which is entirely incorrect. The fix is to use p1.filenode() instead, which returns the parent file node as desired. I don't know how to execute this or make it have a visible effect, so I'm not sure how to test it. It was noticed because asking for the linkrev is an expensive operation when using the remotefilelog extension and this was causing performance regressions with commit.