I know that it’s very old question but if you really need to pass username and password for HTTP basic authentication you can just set helper like this:
git config credential.helper '!f() { sleep 1; echo "username=${GIT_USER}"; echo "password=${GIT_PASSWORD}"; }; f'
UPDATE: I’ve added sleep 1 to the function. In some environments it may be probably needed due to race condition. I’ve got 2 virtual machines running Debian Jessie. They had the same architecture but different CPU and different number of cores. On one of these machines the helper was working fine without sleep. On the other one it wasn’t. After few hours of debugging I run strace to see what’s happening. And it magically started to work. strace just made git a little bit slower.