rebase: update the rebase wrapping to works with hg 4.6
Mercurial core commit bd0086bd3af4 removed _makeextrafn hence breaking our
wrapping. Update the code to wrap rebaseruntime directly in a way that's
compatible with hg 4.1+.
# utility to setup pythonpath to point into the tested repository
export SRCDIR=`dirname $TESTDIR`
if [ -n "$PYTHONPATH" ]; then
export HGTEST_ORIG_PYTHONPATH=$PYTHONPATH
export PYTHONPATH=$SRCDIR:$PYTHONPATH
else
export PYTHONPATH=$SRCDIR
fi