demandimport: patch __builtin__ instead of __builtins__
This helps on implementations other than CPython, where __builtins__ isn't
necessarily defined (as it's an implementation detail for CPython).
#!/bin/sh
echo "[extensions]" >> $HGRCPATH
echo "hgk=" >> $HGRCPATH
hg init repo
cd repo
echo a > a
hg ci -Am adda
hg debug-cat-file commit 0