Correctly handle source_url/target_url with chars needing URL-encoding

Correctly handle source_url/target_url with chars needing URL-encoding

Use urllib.unquote() to URL-decode source_url/target_url values.
All URLs passed to run_svn() should go through svnclient.safe_path()
and we don't want to end-up *double* urllib.quote'ing if the user-
supplied source/target URL's are already URL-encoded or have chars
which need URL-encoding.

* svn2svn/run/svn2svn.py (real_main): Maintain source_url/target_url
as URL-decoded values.
* svn2svn/run/svn2svn.py (gen_tracking_revprops, build_rev_map):
Translate "internal" source_url back to an "external" (URL-encoded)
value.
* svn2svn/svnclient (parse_svn_info_xml): URL-decode 'url' and
'repos_url' values, since all downstream consumers should want these
to be in the "internal" (URL-decoded) format.
* tests/make-ref-repo.sh: Add test-cases to further stress-test
URL-encoding/decoding.