test-phases: test changing null revision phase

The behaviour is correct but for bad reasons: the repo.set() call in
phase command fails for '-1'. It should be rejected explicitely by phase
boundary commands, sadly this is hard to do because phase changes are
not applied atomically.

tests/test-phases.t

@@ -9,6 +9,15 b''
 
   $ hg init initialrepo
   $ cd initialrepo
+
+Cannot change null revision phase
+
+  $ hg phase --force --secret null
+  abort: unknown revision '-1'!
+  [255]
+  $ hg phase null
+  -1: public
+
   $ mkcommit A
 
 New commit are draft by default