Improve pull:merge with more details

|Q            |A  |
|---          |---|
|Bug Fix?     |y  |
|New Feature? |y  |
|BC Breaks?   |n  |
|Deprecations?|n  |
|Tests Pass?  |n  |
|Fixed Tickets|   |
|License      |MIT|
|Doc PR       |   |

- Fix the PR merge-commit title (which was completely wrong).
- PR title now includes the PR-number, type (optional)
  and author (username). When no type is given 'merge' is used instead
- Discussion has all trailing white space stripped
- A commit line only contains the first none-breaking line (subject)
  instead of all the details

Example:

```markdown
merge #13 Testing PR merge with new title (sstok)

This PR was merged into master branch.

Discussion
----------

|Q |A |
|--- |---|
|Bug Fix? |y |
|New Feature? |y |
|BC Breaks? |y |
|Deprecations?|y |
|Tests Pass? |y |
|Fixed Tickets|y |
|License |MIT|
|Doc PR | |

Sent using [Gush](https://github.com/gushphp/gush)

Commits
-------

24e91896a4ab01e6c8bb6c72ffe863157396319d testing commit sstok
```

Sent using [Gush](https://github.com/gushphp/gush)