Consider optionally or permanently adding prune flag to fetch command

[meaghanlewis]

Description

It would be nice if when branches deleted on github.com, they are also deleted in Unity. The current implementation isn't wrong, and I can still manually delete branches in Unity- it just behaves differently than I would expect.

Steps to Reproduce

1. Create a new branch in Unity
2. Make a change on that branch, commit, and push
3. Create a pull request on dotcom
4. Review, merge pull request, and delete branch
5. Go back to Unity
6. Fetch the latest changes

Expected behavior:
The branch deleted on github.com should also be deleted in Unity

Actual behavior:
The branch deleted on github.com remains in the local and remote branches in Unity