Allow commenting on the commit message itself.
Commit messages are as useful as the code changes.
When doing review, we may want to do comment on the commit message itself to ask for more explanation, fix typos, ...
Add the possibility to comment a line in the commit message as it is already done for a line in the diff.
What could be nice is, once #31847 (closed) is done, that the comment on the commit message also starts a discussion (to be resolved) in the MR.