dirstate: use the `changing_files` context in the `keyword` demo This is the way.

dirstate: wrap repository change in appropriate context in `test-context` We need the `wlock` (to add files), the `lock` (to commit), a `transaction` (to commit) and a `changing_files` context (to add files). Strictly speaking, we could let the commit take the `lock` and create a `transaction`, but it seems more consistent that way.

dirstate: use wlock and changing_files context in `test-revlog-ancestry` This is the way.

dirstate: use `dirstate.change_files` to scope the change in `revert` This is the way.

dirstate: use `dirstate.change_files` to scope the change in `gpg` This is the way.

dirstate: use `dirstate.change_files` to scope the change in `tag` This is the way.