profile.d: Reorder the sections in defaults.sh a bit

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
profile.d: Enable Git completion and define a Git prompt, if supported

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
profile.d: Use "." instead of "source" for its shortness

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>