To help track down the setting, I'd try to use:

git config --local credential.helper

git config --global credential.helper

git config --system credential.helper

The first one checks the local repo config, the second is your ~/.gitconfig, and the third is based on where git is installed. Depending on which one comes back showing the credential helper, you can try using the equivalent --unset option:

git config --local --unset credential.helper

git config --global --unset credential.helper

git config --system --unset credential.helper

The last one may not work if you don't have proper permissions. So you may need to run the last one under sudo for it to work correctly. FWIW, you may have installed for the pre-built git images for Mac OS X. If you cat /usr/local/git/etc/gitconfig, you'll see that it does set up the credential helper for you. So the last command above would help fix that problem.