revert: use p2 as parent when reverting against it

revert was always using p1 as parent. This created some minor misbehavior when
reverting against p2. See test change for an example of that.

This is also a useful cleanup for coming refactoring to revert.

mercurial/cmdutil.py

@@ -2250,6 +2250,8 b' def revert(ui, repo, ctx, parents, *pats'
     node = ctx.node()
 
     mf = ctx.manifest()
+    if node == p2:
+        parent = p2
     if node == parent:
         pmf = mf
     else:

tests/test-revert.t

@@ -363,10 +363,8 b' Test revert of a file added by one side '
   R newadd
 
 (revert file added by p2() to p2() state)
-spurious message comes from revert confusion when using p2. To be fixed soon.
 
   $ hg revert -r 'p2()' 'glob:allyou?'
-  reverting allyour
   $ hg status
   M allyour
   M base