Add status buffer inserter for oneline graph

Sometimes it's nice to get a full graph view of recent commits in the
repository you're currently working on. Current status buffer inserters
are available for showing recent commits that *you* are working on but
nothing to just show recent commits in the repository.

Add an inserter that shows `n` commits with

    git log --graph --oneline --decorate --color

Since this inserter is constantly asking for --graph output, optimize
the computation of recent commits. For large repositories it's much more
efficient to do:

    git log HEAD~15

than

    git log --max-count=15

Profiling data is available on #686 (specifically
https://github.com/magit/magit/pull/686#issuecomment-20141371).

---

I guess this is an RFC right now because it doesn't let you select commits. However, I've been using this for a few weeks and it has been incredibly useful in my workflow (I work a lot with other developers so I used to constantly pop open the magit log buffer (which is slow in large repositories)). Anyways, hopefully it's useful to someone else. I'm open for comments on making it mergeable.