tests/test-backwards-remove.t

filelog: raise CensoredNodeError when hash checks fail with censor metadata

With this change, when a revlog revision hash does not match its content, and
the content is empty with a special metadata key, the integrity failure is
assumed to be intentionally caused to remove sensitive content from repository
history.

To allow different Mercurial functionality to handle this scenario differently
a more specific exception is raised than "ordinary" hash failures.

Alternatives to this approach include, but are not limited to:

- Calling a hook when hashes mismatch to allow arbitrary tombstone validation.
  Cons: Irresponsibly easy to disable integrity checking altogether.
- Returning empty revision data eagerly instead of raising, masking the error.
  Cons: Push/pull won't roundtrip the tombstone, so client repos are unusable.
- Doing nothing differently at this layer. Callers must do their own detection
  of tombstoned data if they want to handle some hash checks and not others.
  - Impacts dozens of callsites, many of which don't have the revision data
  - Would probably be missing one or two callsites at any given time
  - Currently we throw a RevlogError, as do 12 other places in revlog.py.
    Callers would need to parse the exception message and/or ensure
    RevlogError is not thrown from any other part of their call tree.

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls
  a