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.
import os
from mercurial import hg, ui
u = ui.ui()
repo = hg.repository(u, 'test1', create=1)
os.chdir('test1')
# create 'foo' with fixed time stamp
f = open('foo', 'w')
f.write('foo\n')
f.close()
os.utime('foo', (1000, 1000))
# add+commit 'foo'
repo[None].add(['foo'])
repo.commit(text='commit1', date="0 0")
print "workingfilectx.date =", repo[None]['foo'].date()