Fix edge-case problem with continue handling for --keep-revnum mode

Fix edge-case problem with continue handling for --keep-revnum mode

* svn2svn/run/svn2svn.py (real_main): Check for --keep-revnum
problem condition inside main loop rather than ahead, so that we
compare the last target_url revision to the *next* source_url rev to
be replayed, not the *last* source_url rev that was replayed.

* svn2svn/run/svn2svn.py (keep_revnum): Use BreakHandler to ensure
that "Committed revision ..." message is always shown if we ran a
keep-revnum mode "svn commit".

* svn2svn/run/svn2svn.py (real_main): Fix "Finished at source
revision ..." message to be more accurate, to display the last
source_url rev replayed. Previously, in some cases (e.g. keep-revnum
mode) this could display the *next* source_url rev to be replayed.