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

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

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

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

commit: use `dirstate.change_files` to scope the associated `addremove` This was significantly more complicated than I expected, because multiple extensions get in the way. I introduced a context that lazily open the transaction and associated context to work around these complication. See the inline documentation for details. Introducing the wrapping transaction remove the need for dirstate-guard (one of the ultimate goal of all this), and slightly affect the result of a `hg rollback` after a `hg commit --addremove`. That last part is deemed fine. It aligns the behavior with what happens after a failed `hg commit --addremove` and nobody should be using `hg rollback` anyway. The small output change in the test come from the different transaction timing and fact the transaction now backup the dirstate before the addremove, which might mean "no file to backup" when the repository starts from an empty state.

commit: move the addremove logic around to make the next changeset clearer Lets do the noise now, without changing any thing. So the new changeset can focus on the actual semantic changes.

largefiles: link the core dirstate._changing context to the lfdirstate one This will be much cleaner and safer to make sure the two dirstates are in sync. This way, the large-files dirstate will simply inherit the state of the main dirstate, so if the core code does the right thing, the large-files code should be right too.

dirstate: add a context for tracking files change Let us start to use it. We will enforce it later.

dirstate: invalidate the dirstate change on transaction failure If the change context lives inside a transaction, the change are not flushed to disk on exit as this is delegated to the transaction. As a result we should also delegate the part that do cleanup on failure. The issue was caught by tests with other change, but it seems useful to fix this as soon as possible.

dirstate: factor the "changing" context logic out This makes it reusable for other types of changes.