Verify-mode

Verify-mode

Add command-line args for verifying content and ancestry, i.e. for
self-testing the replay. Compare the source vs. target history to
make sure we ended-up with equivalent target revisions for each
applicable source revision.

* svn2svn/run/svn2svn.py (verify_commit): Adding.
* tests/make-ref-repo.sh: Add tests for source revisions which
we'll never have an equivalent target revision for, e.g.
svn:mergeinfo property changes.