- Previously, we sorted commits by date, which seemed to work okay.

- The one edge case where this failed was when multiple commits have the same
  commit date (for example: when a range of commits are cherry picked with a
  single command, they all have the same commit date [and different author
  dates]).

- Commits with the same commit date would be sorted arbitrarily, and usually
  break the network graph.

- This commit solves the problem by both sorting by date, and by sorting
  topographically (parents aren't displayed until all their children are
  displayed)

- Include review comments from @adamniedzielski

A more detailed explanation is present here:
https://gitlab.com/gitlab-org/gitlab-ce/issues/30973#note_28706230