Use `svn info` to get working copy root

The existence of `.svn/entries` in a working copy is not a reliable way
under Subversion 1.8. An `svn upgrade` may result in its disappearance.
t: force remove `.svn/entries` file
t: skip without `.svn/entries` test for 1.6
Merge pull request #89 from matthewrmshin/fix-get-wc-root-without-dot-svn-entries

Use `svn info` to get working copy root