mq: do not call [0] on revset

The __getitem__ method have been removed. The "first" method is to be used
instead. Test have been extended to test this code path.

hgext/mq.py

@@ -2001,7 +2001,7 b' class queue(object):'
                 base = repo.changelog.node(rev.first())
                 if base in [n.node for n in self.applied]:
                     raise util.Abort(_('revision %d is already managed')
-                                     % rev[0])
+                                     % rev.first())
                 if heads != [self.applied[-1].node]:
                     raise util.Abort(_('revision %d is not the parent of '
                                        'the queue') % rev.first())

tests/test-mq.t

@@ -1596,4 +1596,11 b' Test interraction with revset (issue4426'
   $ hg ci -m a
   $ hg qimport -r 0::
 
+reimport things
+
+  $ hg qimport -r 1::
+  abort: revision 2 is already managed
+  [255]
+
+
   $ cd ..