Adds aliases glgm, glgb, glgbo

Those aliases are useful when you want to filter / filter out commits in
your feature branches. In particular:

* `glgm`
  Given a merge commit ref, it lists all the commits that where
  merged in. This is useful when you merged a feature branch, and
  then deleted it. With glgm you cal list the commits that were
  merged in from the feature branch.

* `glgb`
  Given a branch ref, it lists all the commits belonging to the
  branch (filtering out merge commits). This is especially useful
  when using the "github flow" workflow.

* `glgbo`
  Like glgb, but it only lists commit belonging to the branch, since
  it branched from master