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.
$ hg init repo
$ cd repo
$ touch foo
$ hg ci -Am 'add foo'
adding foo
$ hg up -C null
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
this should be stored as a delta against rev 0
$ echo foo bar baz > foo
$ hg ci -Am 'add foo again'
adding foo
created new head
$ hg debugindex foo
rev offset length ..... linkrev nodeid p1 p2 (re)
0 0 0 ..... 0 b80de5d13875 000000000000 000000000000 (re)
1 0 13 ..... 1 0376abec49b8 000000000000 000000000000 (re)
$ cd ..