enforce a standard git commit format

[Andrea Ieri]

We should leverage pre-commit to enforce a git commit standard that would include the LP bug the commit is fixing. This would allow us to automate marking bugs as fix-released in launchpad, as well as generating release notes.

Open question: we may sometimes want to add trivial commits that are not tied to a bug. How do we prevent pre-commit from blocking the commit? One possibility would be to have an alternate valid format for trivial commits that includes a specific keyword, e.g. #TRIVIAL

[Xav Paice (xavpaice)]

Suggest having a standard format and some docs surrounding that, e.g. `Fixes bug LP: #12345`, for formatting that is also captured and automatically turned to links by some editors/terminals.

[Giuseppe Petralia (peppepetra)]

This can be done with:

```
  - repo: local
    hooks:
      - id: launchpad-bug
        name: Commit message should contain "Closes-Bug" or "Trivial-Change"
        language: pygrep
        entry: '\A((?!((Closes-Bug: #[0-9]+)|(Trivial-Change))).)*\Z'
        args: [--multiline]
        stages: [commit-msg]
```

Tests: https://pastebin.canonical.com/p/mymT7MgZnt/

Output is also quite explicit re why it is failing

[Andrea Ieri (aieri)]

Two comments:
* some (most?) bugfixes benefit from being split among multiple commits, so we have to also allow Partial-Bug
* in the above scenario, we would ideally want all commits comprising the whole bugfix to be marked as 'Partial-Bug', and the merge commit to contain the 'Closes-Bug' keyword. However, the merge commit is generated by mergebot. How do we enforce this?