topics: add t0 and b0 to the stack t0 or b0 will be the base of the stack and it's the parent of t1 or b1. The cool thing about this is that if you update to t0 using `hg up t0` or do `hg prev` on t1, you will be updated to t0 with the current topic preserved. This patch adds t0 to stack and implement the preserving topic case for t0 while using `hg update`.

# 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