bug#36208: 26.2.50; Add tooling for commit message format checking

[Damien Cassou <damien@cassou.me>]

Hi



writing commit messages that comply with Emacs' guidelines requires a
good understanding of many details described in CONTRIBUTE.  To reduce
the workload of new contributors and of reviewers, part of these
guidelines could be transformed into tools.

Here are some of the checks that humans have to do these days:

1. dots after every sentence (even just "New function" must terminate
   with a dot)
2. double-space after dots ending sentences
3. no indentation (M-q adds 2 spaces which we don't want)
4. line length
5. the Copyright-paperwork-exempt token
6. every single change is documented
7. no colon if another function of the same file has the same comment

I can imagine several tools:

- a major mode for editing commit messages:

  - for check 2., sentence-end-double-space could be set to t

  - for check 3., I guess another variable could be set

  - for check 4., setting fill-column

  - for check 5., a shortcut could help adding such tokens

  - for check 6., a shortcut (beyond `C' which triggers
    `magit-commit-add-log-insert') could add a template with all the
    changes

- a flymake backend to mark problems:

  - for checks 1., 3., 4., and 7., I believe it's obvious

  - for check 2., words ending with a dot and just one space (with a
    whitelist to avoid false positives such as "etc."  and "aka.")

- a patch checker (e.g., `./check_patch.sh *.patch`):

  - could check the same as the flymake backend and also checks 5 and 6.

Best,

-- 
Damien Cassou
http://damiencassou.seasidehosting.st

"Success is the ability to go from one failure to another without
losing enthusiasm." --Winston Churchill