tests/test-issue433.t

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.

http://mercurial.selenic.com/bts/issue433

  $ hg init
  $ echo a > a
  $ hg commit -Ama
  adding a

  $ hg parents -r 0 doesnotexist
  abort: 'doesnotexist' not found in manifest!
  [255]