Diff ACLs and DiffX support are two major features that are in testing

with customers, and are going to be highlights of the 5.0 release.

Still, we want customers on 4.0 to be able to begin experimenting with

these, and have the freedom to alter the APIs if necessary.
This change marks both as experimental.
For DiffXParser, this is just a documentation change, as it's only

used by SCMs that explicitly create it for now (there's no wide-spread

DiffX support across SCMs).
For FileDiffACLHook usage, this is walled off by a diff_acls_feature

feature check, which is marked as experimental by default. Users will

need to enable this feature if they want to try it for now. We'll mark

it as stable in 5.0.

Unit tests pass.