phases: change phase command change detection
A future patch is going to make phase computation lazy, so the phase command can
no longer read and diff the entire phase list directly. This changes the phase
command to build it's own list for diff purposes.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown