Email subject lines are by convention a short explanation of the message. Commit messages, regardless of the SCM, should be the same. The difference with git, and what I inferred from this article, is that commit messages should be verbose. It should be a way for someone to figure out that "bug #33423 has been fixed. Opening files now executes in the proper thread." for a very short message that gets the point across.
If one doesn't have bug tracking software to go to and read about why the commit was done, then it should be much more verbose: "Opening files on a select box executed in the wrong thread. Looks like it erred during startup, where we can't yet fork the process. This patch mitigates the selection box during startup by checking to see if we can fork into our proper thread or not. If we can't, we wait until it becomes available."
There is no reason a commit message cannot be a novel unto itself. And something like that will require a period at the end. If you're trying to limit yourself to a single sentence, or something that can be fit into a tweet, you're only hurting yourself.
