Effects of Sitagliptin (Januvia®) on Blood Glucose Control in Patients With Type 1 Diabetes and CGM Substudy of the Investigator Initiated Study Protocol Effects of Sitagliptin (Januvia) on Glucagon Suppression and GLP-1 Levels in Patients With Type 1 Diabetes; CGM Substudy - ICH GCP - Clinical Trials Registry