pick: register pickstate as an unfinished state
This way pickstate file will indicate that unfinished pick command needs to be
dealt with (--continue or --abort) before modifying the repo. Otherwise it
would be e.g. possible to commit during an interrupted pick and that's not
expected.
test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t