Link Commit to correct upstream documentation (#2936)

`github.Commit.Commit` represents a github-level commit, but the link is
to git-level commits. The semantics and *especially* the metadata are
very different between the two e.g. a github-level commit provides the
*github users* for the author and committer, as well as links to the
github comments.

The github-level commit is a layer around the git-level commit, which is
(partially) provided via the `commit` attribute.

Co-authored-by: Enrico Minack <github@enrico.minack.dev>