Use `revsets.log` as the fallback default revset

[icorbrey]

Description

I'd like jjj to read revsets.log from Jujutsu's configuration and use it as the default revset on startup if jjj.log.default_revset isn't set.

Use Case

This would allow users to be more comfortable with jjj since it'll match their initial config.

Last Steps

• I agree to abide by this project's code of conduct.
• I would like to work on this and submit a PR with my changes.
  (Optional, but contributions are greatly appreciated!)