tests/test-diff-issue2761.t

unionrepo: dynamically create repository type from base repository

This is basically the same thing we just did for bundlerepo except
for union repositories.

.. api::

   ``unionrepo.unionrepository()`` is no longer usable on its own.

   To instantiate an instance, call ``unionrepo.instance()`` or
   ``unionrepo.makeunionrepository()``.

Differential Revision: https://phab.mercurial-scm.org/D4556

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