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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
```

--HG--
branch : stable
5 jobs for topic/stable/heptapod-ci in 4 minutes and 37 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed flake8 #49194

00:00:24

passed tests-py2-cext #49195

00:03:12

passed tests-py2-pure #49196

00:04:12

passed tests-py3-cext #49197

00:04:37

passed tests-py3-pure #49198

00:04:02