tests: test hg status --all with hgext/git

There's a TODO item in git/dirstate.py about obtaining clean files in a more
straightforward way, let's first test that status can and does show clean files
at all.

tests/test-git-interop.t

@@ -142,6 +142,12 b' diff even works transparently in both sy'
    alpha
   +blah
 
+status --all shows all files, including clean:
+  $ hg status --all
+  M alpha
+  ? gamma
+  C beta
+
 Remove a file, it shows as such:
   $ rm alpha
   $ hg status