path: fix win32 relative() when "to" is a prefix

when the basename of "to" was a prefix of the basename of "from" win32
relative() would miss including it in the result

Fixes: https://github.com/nodejs/node/issues/5447
PR-URL: https://github.com/nodejs/node/pull/5456
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Reiss <me@silverwind.io>

lib/path.js

@@ -603,14 +603,28 @@ const win32 = {
     var i = 0;
     for (; i <= length; ++i) {
       if (i === length) {
-        if (lastCommonSep > 2 && // ignore separator match following device root
-            toLen > length &&
-            to.charCodeAt(i) === 92/*\*/) {
-          // We get here if `from` is the exact base path for `to`.
-          // For example: from='C:\\foo\\bar'; to='C:\\foo\\bar\\baz'
-          return toOrig.slice(i + 1);
+        if (toLen > length) {
+          if (to.charCodeAt(toStart + i) === 92/*\*/) {
+            // We get here if `from` is the exact base path for `to`.
+            // For example: from='C:\\foo\\bar'; to='C:\\foo\\bar\\baz'
+            return toOrig.slice(toStart + i + 1);
+          } else if (lastCommonSep === 2) {
+            // We get here if `from` is the device root.
+            // For example: from='C:\\'; to='C:\\foo'
+            return toOrig.slice(toStart + i);
+          }
+        }
+        if (fromLen > length) {
+          if (from.charCodeAt(fromStart + i) === 92/*\*/) {
+            // We get here if `to` is the exact base path for `from`.
+            // For example: from='C:\\foo\\bar'; to='C:\\foo'
+            lastCommonSep = i;
+          } else if (lastCommonSep === 2) {
+            // We get here if `to` is the device root.
+            // For example: from='C:\\foo\\bar'; to='C:\\'
+            lastCommonSep = 3;
+          }
         }
-        lastCommonSep = i;
         break;
       }
       var fromCode = from.charCodeAt(fromStart + i);

test/parallel/test-path.js

@@ -470,7 +470,8 @@ const relativeTests = [
      ['c:/AaAa/bbbb', 'c:/aaaa/bbbb', ''],
      ['c:/aaaaa/', 'c:/aaaa/cccc', '..\\aaaa\\cccc'],
      ['C:\\foo\\bar\\baz\\quux', 'C:\\', '..\\..\\..\\..'],
-     ['C:\\foo\\test', 'C:\\foo\\test\\bar\\package.json', 'bar\\package.json']
+     ['C:\\foo\\test', 'C:\\foo\\test\\bar\\package.json', 'bar\\package.json'],
+     ['C:\\foo\\bar\\baz-quux', 'C:\\foo\\bar\\baz', '..\\baz']
     ]
   ],
   [ path.posix.relative,