tests/test-diff-issue2761.t

bundle2: introduce a ``params`` dictionary on unbundled parts

This exposes all parameters the part received into a ``part.params`` dictionary.
This should be much easier to use.

This dictionary itself does not expose the mandatory or advisory aspect of
parameters, but no current users of bundle2 actually enforce any of this logic.
Coming changesets will improve this aspect.

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