Introduce 'svnancest' utility

Introduce 'svnancest' utility

Introduce a helper utility for calling find_svn_ancestors(), to show
full ancestry ("svn copy" history) for a given starting URL
(optionally at a specific revision).

* svn2svn/run/svnacest.py: Adding
* svn2svn/run/common.py (in_svn, is_child_path, join_path,
  find_svn_ancestors): Move common helper functions to new separate
  library, for sharing with svn2svn/run/svnacest.py.
* svn2svn/svnclient.py (valid_svn_actions): Move this constant here
  from svn2svn/run/svnreplay.py.