In the documentation (Setting up Git repositories - BMC Server Automation 8.9), section 4b refers to leveraging Automation Principles for the GIT credentials as follows:
Specify an automation principal that has the proper credentials for access to the Git repository. You can choose between the following options:
Option Additional information to enter Select an existing automation principal Name of the existing automation principal Create a new automation principal Automation principal credentials — user name, password, and domain
I hope that this helps!