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.

dirstate: introduce a `is_changing_any` property This will embrace other cases than changing parents.

dirstate: rename `pendingparentchange` to `is_changing_parents` This is clearer and more inline witht he other change we did.

dirstate: rename _parentwriters to _changing_level We will have context for changing more than parents, so lets be prepared.