To add a link to a specific commit, simply add commit:<sha-of-commit>
.
So if I were linking to a commit with a SHA of d675f2e5cd6bd87f45246422a79b0c4b93f366c4
, then I would click on "update" and then in the notes section I would put, commit:d675f2e5cd6bd87f45246422a79b0c4b93f366c4
. You can also use the short SHA if you'd like.
You can follow the new link that you just created, which should take you to the repository section that contains that commit (if Redmine is properly set up to integrate with Git, which it sounds like it is). From there you can click "Add related issue" and then enter the bug or feature number. This will show a cool gray box with the commit message next to the bug notes when viewing the bug details.
Hope that helps.