[MERGE] support push without a working tree at the source.

[Aaron Bentley]

Martin Pool wrote:
> On  2 May 2006, John Arbash Meinel <john at arbash-meinel.com> wrote:
> It's just an accident of cmd_diff that it wants a WorkingTree.

Yes, and when I updated cmd_diff to the new API, it was much simpler to
keep that limitation than to try to generalize it to work on
WorkingTrees or Branches, so I decided to limit the work to simply
avoiding deprecated interfaces, and not improving functionality.  I
agree that it should support branches as input.

Aaron