Add a reason why we're xfail-ing this test

This test uses xfail to test the negation of what's described in the
test, i.e. the assertion should "fail" if the command does what we
expect. I think this is a terrible way to write a test, but don't bother
to rewrite. This commit message is left here to tell future maintainers
what the test is all about by git blame.

tests/functional/test_freeze.py

@@ -191,7 +191,12 @@ def test_freeze_svn(script, tmpdir):
 
 
 @pytest.mark.git
-@pytest.mark.xfail
+@pytest.mark.xfail(
+    condition=True,
+    reason="xfail means editable is not in output",
+    run=True,
+    strict=True,
+)
 def test_freeze_exclude_editable(script, tmpdir):
     """
     Test excluding editable from freezing list.