Rewrite GitGraph.js

The current vendored gitgraph.js is no longer maintained and is
difficult to understand, fix and maintain.

This PR completely rewrites its logic - hopefully in a clearer fashion
and easier to maintain.

It also includes @silverwind's improvements of coloring the commit dots
and preventing the flash of incorrect content.

Further changes to contemplate in future will be abstracting out of the
flows to an object, storing the involved commit references on the flows
etc. However, this is probably a required step for this.

Replaces #12131
Fixes #11981 (part 3)

Signed-off-by: Andrew Thornton <art27@cantab.net>