Sort the network graph both by commit date and topographically

What does this MR do?

• Previously (!10936 (merged)), we sorted commits for the network graph 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)

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

Are there points in the code the reviewer needs to double check?

• The spec added in graph_spec runs in quadratic time, but I thought this was okay because it deals with a small number of items. Please feel free to disagree. :)
• Are there any cases where the network graph might break even with this dual sorting strategy?

Screenshots (if relevant)

Before

After

What are the relevant issue numbers?

Closes #30973 (closed)

Tasks

• Investigation
  • Reproduce the issue
  • Does it happen on 9.0.0 as well
  • What about earlier versions?
• Implementation
• Tests
  • Added
  • Passing
• Meta
  • CHANGELOG entry created
  • API support added
  • Branch has no merge conflicts with master
  • Squashed related commits together
  • Added screenshots
  • Check for clean merge with EE
  • Documentation added/updated
• Review
  • Reviewer
  • Maintainer
• Wait for merge