reparse fixes

Properly track modification revisions of imported files and selectively apply ForceUpdate.

The former is a correctness fix which improves update tracking. I.e. previously the code sometimes decided not to reparse a file even though it has changed.

The latter is a huge performance improvement for files with many include files. Switching branches or apply git stash triggered essentially a force-recursive update, which is super slow. Now instead only the changed files will be updated.