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

hg init
echo 123 > a
hg add a
hg commit -m "first" -d "0 0" a
mkdir sub
echo 321 > sub/b
hg add sub/b
hg commit -m "second" -d "0 0" sub/b
cat sub/b
hg co 0
cat sub/b
ls sub

true