Add a hg hook that references commits in issues

[fhd]

Background

Whenever we currently commit something linked to a specific issue, we manually add a comment pointing to that commit. That's fairly easy to automate, since commit messages always have the same format.

We can extend this further in the future:

• We could reject commits that don't specify either an issue or Noissue in the canonical format.
• We could automatically close issues once they've been mentioned in a commit.

But all that is follow-up issue material, we should keep it simple at first.

What to change

Write a changegroup hook that inspects the changeset descriptions (i.e. commit messages) and checks if an issue was referenced. The canonical format for that is: Issue [number] - [message].

If an issue was referenced, the hook should post a comment to the respective issue, posting the changeset URL. The message should be the following if a single commit referencing an issue was found:

A commit referencing this issue has landed:
CHANGESET_URL

And the following if multiple commits referencing an issue were found:

Some commits referencing this issue have landed:

• CHANGESET_URL_1
• CHANGESET_URL_2

[fhd]

Will probably end up in infrastructure rather than sitescripts because an internal hook makes more sense. But the jury is still out I suppose.

[fhd]

Silly me, internal hooks are just modules somewhere on the PYTHONPATH, sitescripts it is.

Also updating the description a bit, I want to keep it simple for now.