revsets: let parents() return parents of working dir

This patch makes the 'set' argument to revset function parents() optional.
Like p1() and p2(), if no argument is given, returns the parent(s) of the
working directory.

Morally equivalent to 'p1()+p2()', as expected.

mercurial/revset.py

@@ -232,9 +232,13 b' def p2(repo, subset, x):'
     return [r for r in subset if r in ps]
 
 def parents(repo, subset, x):
-    """``parents(set)``
-    The set of all parents for all changesets in set.
+    """``parents([set])``
+    The set of all parents for all changesets in set, or the working directory.
     """
+    repo.ui.debug(repr(x), '\n')
+    if x is None:
+        return [r.rev() for r in repo[x].parents()]
+
     ps = set()
     cl = repo.changelog
     for r in getset(repo, range(len(repo)), x):

tests/test-revset-dirstate-parents.t

@@ -21,6 +21,7 b''
 null revision
   $ log 'p1()'
   $ log 'p2()'
+  $ log 'parents()'
 
 working dir with a single parent
   $ echo a > a
@@ -28,6 +29,8 b' working dir with a single parent'
   $ log 'p1()'
   0
   $ log 'p2()'
+  $ log 'parents()'
+  0
 
 merge in progress
   $ echo b > b
@@ -40,3 +43,6 b' merge in progress'
   2
   $ log 'p2()'
   1
+  $ log 'parents()'
+  2
+  1