Not possible to fast-forward even with no changes

Your commits on your 'develop' branch do not match the commits on your 'origin' branch. Do this:

git pull origin develop --rebase

This happens when (a) you committed something on that branch earlier, or (b) the history on the remote server changed in a non-standard way (this shouldn't normally happen but repository owners sometimes don't play by the rules).

For the following I'm assuming you're on foo and the upstream branch is origin/foo.

• Use git log ..origin/foo to see what commits are new on the remote side.
• Use git log origin/foo.. to see what commits exist on your side that don't exist on the remote side (this will show you any commits that are preventing fast-forwarding).
• If you conclude that those commits are not needed or already present in a different form on the remote side, git reset --hard origin/foo to force your branch to become equal to the remote one (this will destroy all uncommitted changes and any commits not contained in remote/foo will become unreachable).