Fix cherry picks for forks

What does this MR do and why?

Currently, cherry picking a commit from a fork fails if the commit does not exist upstream. In order to fix it, we need to pre-fetch the commit into the target repo.

Related issues:

• `cherry-pick` across forks doesn't work (#361363 - closed) (contains steps to reproduce)
• "Cherry-pick into a project" feature always ret... (#330312 - closed)