I think the problem was caused because:

1. I accidentally committed something (let's call it A) on live instead of master.
2. I then committed something else (B say) on master.
3. I realised I made the mistake 1. and merged live onto master.
4. I still needed to merge B onto live, so I merged master onto live.

I think this created a "crossing" of commit paths that makes it impossible for git merge to fast-forward.

The solution was to first ensure that the two branches really did have the same contents:

git diff master..live git cherry -v master live git cherry -v live master 

all yielded no result. Then I changed live to point at exactly the same commit as master:

git branch -f live ae5e70... # Hash of the commit pointed to by master git push --force origin live:live 

Now I can fast-forward again. Note that this can be dangerous and should probably not be done when the repository is shared.

I think the problem was caused because:

1. I accidentally committed something (let's call it A) on live instead of master.
2. I then committed something else (B say) on master.
3. I realised I made the mistake 1. and merged live onto master.
4. I still needed to merge B onto live, so I merged master onto live.

I think this created a "crossing" of commit paths that makes it impossible for git merge to fast-forward.

The solution was to first ensure that the two branches really did have the same contents:

git diff master..live git cherry -v master live git cherry -v live master 

all yielded no result. Then I changed live to point at exactly the same commit as master:

git branch -f live ae5e70... # Hash of the commit pointed to by master git push --force origin live:live 

Now I can fast-forward again. Note that this can be dangerous and should probably not be done when the repository is shared.