Describe the feature or problem you’d like to solve

Currently syncing a fork is a multiple step process, described in the GitHub help docs:

https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/syncing-a-fork

Once other fork related features have been added to cli it would be great to have a command to sync your fork.

Syncing a fork is often a challenging process for new contributors and forkers, this could make it a lot easier for them to get started and stay up to date.

Proposed solution

Add a command that allows you to pull upstream changes and merging them. With the context of GitHub behind it, the command could default to using the direct parent of the fork as the upstream, but could perhaps have an optional parameter to specify a different upstream or sibling fork.

Additional context