forester: Fetch latest changes from remotes

[koooosh]

Description:

After seeding a forest, there's currently no way to fetch new commits from the upstream remotes without manually running git
commands in each bare repo.

Problem

When working with a forest over time, the upstream repositories receive new commits. Users need a way to pull these changes into
their local bare repos so they can:

• Create new groves with the latest code
• See what's changed upstream before syncing their work