Fixes T4682. If SVN 1.7 is detected (using the prop_index var) making sure existing props are added to both old and new and only + get added to new and - to old. Added a test case for this as well that shows both adding and removing properties from svn:ignore.

New test case added, existing tests for this code pass. The one failure failed before code changes, so assume it doesn't pass in my environment, or not at all. Also tested in my local copy of Arcanist against the same diff that is in T4682.