as there's actually no "brief" format originally in git (by "originally"
I mean the ones listed for `--format=<format>` in `man git-log`).
Also, we already fit in `glG` all the information that `glb` had.
So add `glO`, that is a one-line variant of `glG`.
Now we get nice symmetrical formats in `glg`/`glG` and in `glo`/`glO`.