branching: merge stable into default

branching: merge stable into default

branchcache: use update_disk to refresh 'served' and 'served.hidden' The `update_disk` method is dedicated to this kind of usecase. Now that the writting patterns are more consistent, we can use it to warm these two important cache. I am dropping the first comment about "refreshing all the others" because it is false. If a branchmap already exist for "served", none of the subset will be updated.

branchcache: explictly update disk state only if no transaction exist If a transaction exist the `write_dirty` call will eventually be done and the state will be synched on disk. It is better to no interfer with that.

branchcache: do not use `__getitem__` in updatecache The `update_disk` method uses `updatecache` and the point of `update_disk` is to be able to do alternative processing to the one we do in `__getitem__`. So we calling `__getitem__` in `updatecache` defeat this purpose. Instead we do the equivalent explicitly to preserve the spirit of `update_disk` (that we will actually put to use soon, I promise)

branchcache: explicitly track inheritence "state" We move from a binary "dirty" flag to a three value "state": "clean", "inherited", "dirty". The "inherited" means that the branch cache is not only "clean", but it is a duplicate of its parent filter. If a branch cache is "inherited", we can non only skip writing its value on disk, but it is a good idea to delete any stale value on disk, as those will just waste time (and possibly induce bug) in the future. We only do this in the update related to transaction or explicit cache update (e.g `hg debugupdatecache`). Deleting the file when we simply detected a stall cache during a read only operation seems more dangerous. We rename `copy` to `inherit_for` to clarify we associate a stronger semantic to the operation.