Add support for `--graph` option in `dolt log`

[nacho53dolt]

One of my favourite git commands in git is git log --oneline --graph, which shows really effectively how all the different merges/pull requests/commits/branches fit together.

Currently, the --graph option is not supported in dolt. I know it may not be a high priority, but it is ultimately something I'd love to see.

This issue is a prerequisite issue for #4357 (dolt la alias fails).