Right now, ArX maintains pristine trees alongside the project tree.  This is fragile, because these files can be changed while the archive is still intact.  So one solution is to simply make the latest archive revision a complete source tree instead of a tarball.  In conjunction with integrity checks, this would ensure that, for example, "what-changed" was always diffing the right files.  When making branches, we could use hard links to reduce wasted space.

This would cause problems if you normally have a remote archive that you only connect to when you commit.  It would also require smarter servers, as in bug #4787.