dirstate: use `reset_state` to drop file in `update_file_p1`
The `dropfile` function is on its way out.
Differential Revision: https://phab.mercurial-scm.org/D11439
dirstate: support file tracked nowhere in `reset_state`
This let the dirstatemap decide when to drop files.
Differential Revision: https://phab.mercurial-scm.org/D11438
dirstate: update the documentation of the dirstatemap API
The API changed so should the documentation.
Differential Revision: https://phab.mercurial-scm.org/D11436