git: Your branch is ahead by X commits

Use

git pull --rebase

The --rebase option means that git will move your local commit aside, synchronise with the remote and then try to apply your commits from the new state.

If you get this message after doing a git pull remote branch, try following it up with a git fetch. (Optionally, run git fetch -p to prune deleted branches from the repo)

Fetch seems to update the local representation of the remote branch, which doesn't necessarily happen when you do a git pull remote branch.