Offer to fetch and merge new upstream contents when fork is behind

Problem to solve

In order to keep my fork up to date, I have to merge contents from the upstream. This is very manual and time consuming.

Proposal

Once we alert users their fork is behind from the parent (#330242) we should offer to automatically fetch and merge the contents of the parent into the fork as long as there are no conflicts.

Alternatives

Described in https://forum.gitlab.com/t/refreshing-a-fork/32469/2?u=dnsmichi

Local sync with a cloned fork on the CLI

git remote add upstream https://gitlab.com/upstream_user_group/project.git || true && git fetch upstream && git checkout main && git pull upstream main && git push origin main

Repository mirroring (Pull)

Use the upstream repository as source. That's a Premium+ feature.

https://docs.gitlab.com/ee/user/project/repository/repository_mirroring.html#pulling-from-a-remote-repository