Fix relative Windows path normalization (#1261)

Fixes #1223

libc/calls/mkntpath.c

@@ -55,6 +55,19 @@ textwindows size_t __normntpath(char16_t *p, size_t n) {
       // matched "/../" or "/..$"
       while (j && p[j - 1] == '\\')
         --j;
+      if (j && p[j - 1] == '.') {
+        // matched "." before
+        if (j >= 2 && p[j - 2] == '.' &&  //
+            (j == 2 || p[j - 3] == '\\')) {
+          // matched "^.." or "/.." before
+          p[++j] = '.';
+          ++j;
+          continue;
+        } else if (j == 1 || p[j - 2] == '\\') {
+          // matched "^." or "/." before
+          continue;
+        }
+      }
       while (j && p[j - 1] != '\\')
         --j;
     } else {

test/libc/calls/mkntpath_test.c

@@ -51,4 +51,34 @@ TEST(mkntpath, testRemoveDoubleSlash) {
   EXPECT_STREQ(u"C:\\Users\\jart\\.config", p);
 }
 
+TEST(mkntpath, testRelativeCurrentParent) {
+  EXPECT_EQ(3, __mkntpath("./../", p));
+  EXPECT_STREQ(u"..\\", p);
+}
+
+TEST(mkntpath, testRelativeParentParent) {
+  EXPECT_EQ(6, __mkntpath("../../", p));
+  EXPECT_STREQ(u"..\\..\\", p);
+}
+
+TEST(mkntpath, testRelativeParentParentParent) {
+  EXPECT_EQ(9, __mkntpath("../../../", p));
+  EXPECT_STREQ(u"..\\..\\..\\", p);
+}
+
+TEST(mkntpath, testRelativeDirParent) {
+  EXPECT_EQ(2, __mkntpath("abc/../", p));
+  EXPECT_STREQ(u".\\", p);
+}
+
+TEST(mkntpath, testRelativeDirCurrent) {
+  EXPECT_EQ(4, __mkntpath("abc/./", p));
+  EXPECT_STREQ(u"abc\\", p);
+}
+
+TEST(mkntpath, testRelativeDirDirParent) {
+  EXPECT_EQ(4, __mkntpath("abc/def/../", p));
+  EXPECT_STREQ(u"abc\\", p);
+}
+
 #endif /* SupportsWindows() */