status: process from_p2 file the same as merged one
What matters here is that the file constains information coming from the second
parent and should be considered modified on plain `hg status.
So we can process from_p2 file sooner. It also highlight that we probably
don't need the merged/from_p2 distinction at higher level.
Differential Revision: https://phab.mercurial-scm.org/D11591