heptapod-ci: add a script to map evolve branch to mercurial revision for tests
map-hg-rev.sh can be tested using this command:
```sh
for i in $(hg log -T '{branch}\n' | sort -u); do
echo $i $(testlib/map-hg-rev.sh $i)
done
```
# 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