ci: Add commit message linting commit hook

Add `gitlint` to our `pre-commit` hooks. `gitlint` runs as a
`commit-msg` hook. To run `gitlint` you can use one of the following
methods:

* Install as a commit message hook: `pre-commit install --hook-type
  commit-msg`

* Run through `pre-commit` CLI on the latest commit: `pre-commit run
  gitlint --hook-stage commit-msg --commit-msg-filename
  .git/COMMIT_EDITMSG`

If installing the commit hook, you must run the `pre-commit` install
command above even if you've already run a plain `pre-commit install`.
`pre-commit install` will not install `commit-msg` hooks by default.

We can't use `pre-commit` to run `gitlint` in CI, because the default
`pre-commit` hook for `gitlint` forces users to pass the
`commit-msg-filename` argument, which has the following drawbacks:

* We can only lint the latest commit (`.git/COMMIT_EDITMSG`) or all
  commits in the history (`.git/HEAD`) -  we can't specify an arbitrary
  range of commits we want to lint. We can't lint all commits in the
  history in this repo, because some of them fail the `gitlint` checks.

* `COMMIT_EDITMSG` isn't created in the CI env: it's an artifact
  created by git after editing the commit message.

For those reasons we just run `gitlint` directly in CI on the range of
commits from the target branch to HEAD, e.g. `origin/main..HEAD` for MRs
targeting the main branch.

Signed-off-by: Robert Walton <rob.walton@arm.com>
Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>