[PATCH] Add ability to mark change sets as merged

This is effectively a way to manipulate the merge memory stored in the
"svnmerge-integrated" property. It's the flip-side of the 'block'
concept.

Use case: A change set which is sufficiently different when backported
that use of 'svn merge' is no longer appropriate. Example scenarios:

   o The actual change you want to apply to branch has no overlap with
   its incarnation on the source branch, yet is conceptually
   equivalent.

  o Only a subset of a change set warrants application.

  o The branch content has drifted far enough apart to make automatic
  merging impossible.

Typical usage is paired with manually-issued, targeted 'svn merge'
and/or 'svn diff' commands, stand-alone 'diff' and 'patch', and/or
manual edits to the target branch.

-- 
Daniel Rall
p.s. On a related note, I've got 'svnmerge.py rollback [--memory]'
stubbed out too.
Incidently, these changes are to implement the "Record Manual Merge"
and "Rollback Merge" requirements mentioned in Subversion's Merge
Tracking Requirements doc
<http://subversion.tigris.org/merge-tracking/requirements.html>