Fix path calculation in file reversion

[abingham]

Currently when we attempt to revert files after changes, we are using incomplete paths. That is, we extract the paths from changesets and try to revert those, but those are generally relative paths, and they're generally relative to the project root. This means that, unless we happen to be doing reversion from the project root (uncommon!), the revert doesn't work because we resolve to a null buffer.

I think the simple answer is that we need to prepend the project root to each path before trying to get its buffer.