dirstate: rename forget to drop
It has substantially different semantics from forget at the command
layer, so change it to avoid confusion.
We can't simply combine it with remove because we need to explicitly
drop non-added files in some cases like commit.
$ hg init t
$ cd t
$ echo a > a
$ hg add a
$ hg commit -m test
$ rm .hg/requires
$ hg tip
abort: index 00changelog.i unknown format 2!
[255]
$ echo indoor-pool > .hg/requires
$ hg tip
abort: requirement 'indoor-pool' not supported!
[255]