orgit-log-store: Use branch name, not 'HEAD'

As you guessed in #1, orgit works with Magit's master. When trying this out on master, I noticed that orgit-log points to HEAD instead of a branch name (like it does with on next). There might be a motivation or use case that I'm not thinking of, but pointing to a branch name seems like a better default for a log link. What do you think?

Also, I've pushed a next branch to kyleam/orgit with the commit from #1 and an additional change for using orgit with Magit's next.