evolve: run in a single transaction
This speeds up `hg evolve --all` of a stack of 24 commits in the hg
repo from 11.5s to 3.7s (3x). As can be seen in the tests, it also
avoid some warnings about transiently unstable commits.
# 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