status: add support for log-like template keywords and functions
It's bound to ctx2 since "hg status" can be considered to show the status
of the files at ctx2 given ctx1 as the base.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a