If you're going to use git, you should use the power git offers... branches!
On the assumption that you don't want to complicate the workflow any more than necessary though, simply tell your developer that before committing, always git pull --rebase
. In a nutshell, this will: a) save off your local changes, b) pull remote changes (that is, changes others have committed and pushed), c) replay your local changes onto the new code base.