The merge-upstream route was so far performing any kind of merge, even
those that would create merge commits and thus make your branch diverge
from upstream, requiring manual intervention via the git cli to undo the
damage.
With the new optional parameter ff_only, we can instruct gitea to error
out, if a non-fast-forward merge would be performed.
Fix#33271
Suppose there is a `branch-a` in fork repo:
1. if `branch-a` exists in base repo: try to sync `base:branch-a` to `fork:branch-a`
2. if `branch-a` doesn't exist in base repo: try to sync `base:main` to `fork:branch-a`
