Improve PR body truncation

[rarkins]

If release notes are too long, then we truncate them arbitrarily. Ideally we could do this more cleverly, especially when there are multiple packages in a PR.

[pi0]

This is in the continuation of renovatebot/presets#85 (comment).

The babel release notes are huge and meanwhile, there's a limit to how many characters you can fit in a GitHub PR body, so a lot had to be truncated. Notice there's no "Created by Renovate Bot" footer there either. Anyway right now we do the simplest approach and simply truncate PR body text starting from the beginning, but in this case, it's meant only babel can fit.

My suggested behaviour is to extract big changelogs into additional replies and maintain them in case of edits.

Create a PR:

• 1. Gather changelogs
• 2. Scan through and change the text of longer ones intro a short message/link like "See below"
• 3. For each excluded item in step (ii), add an additional message with an special HTML comment

Modify a PR:

• 1. Do steps (i) and (ii)
• 2. Read all previous replies for special HTML comment
• 3. Do step (iii) but only edit posts if already added in an additional reply

Now we can also truncate big replies and add a link to the original changelog.

[rarkins]

@pi0 thanks for the ideas. Indeed, expanding the existing PR body to extend it into 1 or more additional "comments" is an alternative to truncating. In fact at one point I had considered whether to move all embedded changelogs out of the main PR body for those who like to merge squash.

[rarkins]

I've also added a small tweak just now to make sure we don't truncate the PR's configuration and footer and instead only truncate the "middle" release notes section.

[viceice]

Moving changelogs to comments is a good idea, like that.