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.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
revision 0
branch default
phase draft
adda
$ cd ..