match: add missing "return set()", add FIXME to test to doc a bug

These were both brought up during the codereview of D4130.

Differential Revision: https://phab.mercurial-scm.org/D4160

mercurial/match.py

@@ -910,6 +910,7 b' class prefixdirmatcher(basematcher):'
             return self._matcher.visitchildrenset(dir[len(self._pathprefix):])
         if dir in self._pathdirs:
             return 'this'
+        return set()
 
     def isexact(self):
         return self._matcher.isexact()

tests/test-match.py

@@ -97,6 +97,8 b' class PatternMatcherTests(unittest.TestC'
         self.assertEqual(m.visitchildrenset('.'), 'this')
         self.assertEqual(m.visitchildrenset('dir/subdir/x'), set())
         self.assertEqual(m.visitchildrenset('folder'), set())
+        # FIXME: These should probably be {'subdir'} and 'this', respectively,
+        # or at least 'this' and 'this'.
         self.assertEqual(m.visitchildrenset('dir'), set())
         self.assertEqual(m.visitchildrenset('dir/subdir'), set())