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.
$ HGENCODING=utf-8
$ export HGENCODING
$ try() {
> hg debugrevspec --debug $@
> }
$ log() {
> hg log --template '{rev}\n' -r "$1"
> }
$ hg init repo
$ cd repo
$ try 'p1()'
('func', ('symbol', 'p1'), None)
-1
$ try 'p2()'
('func', ('symbol', 'p2'), None)
null revision
$ log 'p1()'
$ log 'p2()'
$ log 'parents()'
working dir with a single parent
$ echo a > a
$ hg ci -Aqm0
$ log 'p1()'
0
$ log 'p2()'
$ log 'parents()'
0
merge in progress
$ echo b > b
$ hg ci -Aqm1
$ hg up -q 0
$ echo c > c
$ hg ci -Aqm2
$ hg merge -q
$ log 'p1()'
2
$ log 'p2()'
1
$ log 'parents()'
2
1