tests/test-diff-issue2761.t

clfilter: introduce `filteredrevs` attribute on changelog

This changeset allows changelog object to be "filtered". You can assign a set of
revision numbers to the `changelog.filteredrevs` attributes. The changelog will
then pretends these revision does not exists in this repo.

A few methods need to be altered to achieve this behavior:

- tip
- __iter_
- irevs
- hasnode
- headrevs

For consistency and to help debugging, the following methods are altered too.
Tests tend to show it's not necessary to alter them but have them raise proper
exception helps to detect bad acces to filtered revisions.

- rev
- node
- linkrev
- parentrevs
- flags

The following methods would also need alteration for consistency purpose but
this is non-trivial and not done yet.

- nodemap
- strip

The C version of headrevs is not run if there is any revision to filter. It'll
need a proper rewrite later to restore performance.

Test issue2761

  $ hg init

  $ touch to-be-deleted
  $ hg add
  adding to-be-deleted
  $ hg ci -m first
  $ echo a > to-be-deleted
  $ hg ci -m second
  $ rm to-be-deleted
  $ hg diff -r 0

Same issue, different code path

  $ hg up -C
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ touch does-not-exist-in-1
  $ hg add
  adding does-not-exist-in-1
  $ hg ci -m third
  $ rm does-not-exist-in-1
  $ hg diff -r 1