revset: add support for p2(wdir()) to get second parent of working directory

This adds support for finding the second parent of working directory using the
p2 predicate.

mercurial/revset.py

@@ -1434,7 +1434,12 b' def p2(repo, subset, x):'
     ps = set()
     cl = repo.changelog
     for r in getset(repo, fullreposet(repo), x):
-        ps.add(cl.parentrevs(r)[1])
+        try:
+            ps.add(cl.parentrevs(r)[1])
+        except error.WdirUnsupported:
+            parents = repo[r].parents()
+            if len(parents) == 2:
+                ps.add(parents[1])
     ps -= {node.nullrev}
     # XXX we should turn this into a baseset instead of a set, smartset may do
     # some optimizations from the fact this is a baseset.

tests/test-revset.t

@@ -1233,6 +1233,7 b' Test working-directory revision'
   2147483647
   $ hg debugrevspec 'p1(wdir())'
   7
+  $ hg debugrevspec 'p2(wdir())'
   $ hg debugrevspec 'parents(wdir())'
   7
   $ hg debugrevspec 'wdir()^1'