path: fix verbose relative() output

Fixes: https://github.com/nodejs/node/issues/5383
PR-URL: https://github.com/nodejs/node/pull/5389
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Evan Lucas <evanlucas@me.com>

lib/path.js

@@ -1282,6 +1282,10 @@ const posix = {
           // We get here if `from` is the exact base path for `to`.
           // For example: from='/foo/bar'; to='/foo/bar/baz'
           return to.slice(i + 2);
+        } else if (fromLen > length && from.charCodeAt(i + 1) === 47/*/*/) {
+          // We get here if `to` is the exact base path for `from`.
+          // For example: from='/foo/bar/baz'; to='/foo/bar'
+          lastCommonSep = i;
         }
         break;
       }

test/parallel/test-path.js

@@ -481,7 +481,8 @@ const relativeTests = [
      ['/var/lib', '/var/apache', '../apache'],
      ['/var/', '/var/lib', 'lib'],
      ['/', '/var/lib', 'var/lib'],
-     ['/foo/test', '/foo/test/bar/package.json', 'bar/package.json']
+     ['/foo/test', '/foo/test/bar/package.json', 'bar/package.json'],
+     ['/Users/a/web/b/test/mails', '/Users/a/web/b', '../..']
     ]
   ]
 ];