D4365: match: make exactmatcher.visitchildrenset return file children as well

[Yuya Nishihara]

> +        if not self._fileset or dir not in self._dirs:

Just curious why you've added `not self._fileset`.

Except a vast style change, the important part is just this:

```
@@ -590,7 +590,7 @@
         if dir not in self._dirs:
             return set()
 
-        candidates = self._dirs - {'.'}
+        candidates = self._fileset | self._dirs - {'.'}
         if dir != '.':
             d = dir + '/'
             candidates = set(c[len(d):] for c in candidates if
```