phases: track phase changes from 'retractboundary' We adds new computation to find and record the revision affected by the boundary retraction. This add more complication to the function but this seems fine since it is only used in a couple of rare and explicit cases (`hg phase --force` and `hg qimport`). Having strong tracking of phase changes is worth the effort.

% creating repo
% add and commit
C test.py
% change
M test.py