Add option for importing a course from a named branch of a git repo

(cherry picked from commit 91305b84f9793fed40926d6a1f322ecbf3477f46)

Conflicts:
	lms/djangoapps/dashboard/management/commands/tests/test_git_add_course.py
Add option for importing a course from a named branch of a git repo

(cherry picked from commit 91305b84f9793fed40926d6a1f322ecbf3477f46)

Conflicts:
	lms/djangoapps/dashboard/management/commands/tests/test_git_add_course.py