path: fix win32.relative() for some Unicode paths

lib/path.js

@@ -437,15 +437,26 @@ const win32 = {
     if (from === to)
       return '';
 
-    const fromOrig = win32.resolve(from);
-    const toOrig = win32.resolve(to);
+    let fromOrig = win32.resolve(from);
+    let toOrig = win32.resolve(to);
 
     if (fromOrig === toOrig)
       return '';
 
     from = fromOrig.toLowerCase();
     to = toOrig.toLowerCase();
 
+    if (fromOrig.length !== from.length) {
+      fromOrig = fromOrig.normalize('NFD');
+      from = fromOrig.toLowerCase();
+    }
+    let toNormalized;
+    if (toOrig.length !== to.length) {
+      toNormalized = true;
+      toOrig = toOrig.normalize('NFD');
+      to = toOrig.toLowerCase();
+    }
+
     if (from === to)
       return '';
 
@@ -493,18 +504,24 @@ const win32 = {
     // return the original `to`.
     if (i !== length) {
       if (lastCommonSep === -1)
-        return toOrig;
+        return (toNormalized ? toOrig.normalize('NFC') : toOrig);
     } else {
       if (toLen > length) {
         if (to.charCodeAt(toStart + i) === CHAR_BACKWARD_SLASH) {
           // 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);
+          return (
+            toNormalized ?
+              toOrig.slice(toStart + i + 1).normalize('NFC') :
+              toOrig.slice(toStart + i + 1));
         }
         if (i === 2) {
           // We get here if `from` is the device root.
           // For example: from='C:\\'; to='C:\\foo'
-          return toOrig.slice(toStart + i);
+          return (
+            toNormalized ?
+              toOrig.slice(toStart + i).normalize('NFC') :
+              toOrig.slice(toStart + i));
         }
       }
       if (fromLen > length) {
@@ -535,12 +552,20 @@ const win32 = {
 
     // Lastly, append the rest of the destination (`to`) path that comes after
     // the common path parts
-    if (out.length > 0)
-      return `${out}${toOrig.slice(toStart, toEnd)}`;
+    if (out.length > 0) {
+      const slice = (
+        toNormalized ?
+          toOrig.slice(toStart, toEnd).normalize('NFC') :
+          toOrig.slice(toStart, toEnd));
+      return `${out}${slice}`;
+    }
 
     if (toOrig.charCodeAt(toStart) === CHAR_BACKWARD_SLASH)
       ++toStart;
-    return toOrig.slice(toStart, toEnd);
+    return (
+      toNormalized ?
+        toOrig.slice(toStart, toEnd).normalize('NFC') :
+        toOrig.slice(toStart, toEnd));
   },
 
   toNamespacedPath(path) {

test/parallel/test-path-relative.js

@@ -31,7 +31,12 @@ const relativeTests = [
      ['\\\\foo\\baz-quux', '\\\\foo\\baz', '..\\baz'],
      ['\\\\foo\\baz', '\\\\foo\\baz-quux', '..\\baz-quux'],
      ['C:\\baz', '\\\\foo\\bar\\baz', '\\\\foo\\bar\\baz'],
-     ['\\\\foo\\bar\\baz', 'C:\\baz', 'C:\\baz']
+     ['\\\\foo\\bar\\baz', 'C:\\baz', 'C:\\baz'],
+     ['c:\\a\\İ', 'c:\\a\\İ\\test.txt', 'test.txt'],
+     ['c:\\İ\\a\\İ', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'],
+     ['c:\\İ\\a\\i̇', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'],
+     ['c:\\i̇\\a\\İ', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'],
+     ['c:\\ß\\a\\ß', 'c:\\ß\\b\\ß\\test.txt', '..\\..\\b\\ß\\test.txt']
     ]
   ],
   [ path.posix.relative,