Fixes to "hg heads -r FOO": Make it actually work (undefined variable 'rev'; allow to pass a rev parameter). repo.branchlookup() doesn't need a copy of heads because it doesn't modify it. Use None as default argument to heads() instead of nullid. Doc string PEPification.

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "0 0"
hg verify
hg parents
hg status
hg undo
hg verify
hg parents
hg status