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
```
#!/bin/sh
set -eu
case $1 in
default)
echo default
;;
mercurial-*)
exp=$(echo "$1" | cut -d- -f2 | sed 's#\.#\\.#g')
echo 'max(tag("re:^'"$exp"'"))'
;;
*)
echo stable
;;
esac