Fix incremental skip regression from modify-window nsec comparison

metadata_mtime_matches() required exact nanosecond equality at the default
modify_window=0, but destination write-time nsecs never match source
creation-time nsecs unless -M preserves metadata. Compare whole seconds
(rsync's default quick-check) at window 0; keep the subsecond-refined
tolerance for explicit window values.

src/shared/metadata.c

@@ -43,6 +43,8 @@ bool metadata_mtime_matches(time_t left_sec, long left_nsec, time_t right_sec, l
     seconds--;
     nanoseconds += 1000000000LL;
   }
+  if (modify_window == 0)
+    return left == right;
   return seconds < modify_window || (seconds == modify_window && nanoseconds == 0);
 }
 

tests/test_metadata.c

@@ -131,7 +131,7 @@ static void test_metadata_mtime_window() {
   EXPECT_FALSE(metadata_mtime_matches(100, 100000000, 102, 900000000, 2));
   EXPECT_TRUE(metadata_mtime_matches(100, 900000000, 102, 900000000, 2));
   EXPECT_FALSE(metadata_mtime_matches(100, 900000000, 101, 100000001, 0));
-  EXPECT_FALSE(metadata_mtime_matches(100, 100000000, 100, 100000001, 0));
+  EXPECT_TRUE(metadata_mtime_matches(100, 100000000, 100, 100000001, 0));
   EXPECT_TRUE(metadata_mtime_matches(100, 100000000, 100, 100000000, 0));
 }