salvaged: record salvaged in ChangingFiles at commit time
The new code is a simple but effective way to detect this information. We might
be able to move it inside the various conditionnal above, but I want to focus
on simplicity until we have a full working stack.
It is worth noting that if we record the information in the ChangingFiles
object, it is not persisted yet. This will comes with later changesets.
Differential Revision: https://phab.mercurial-scm.org/D9120