verboseSuperprojectUpdate was a boolean config option that either do not
include anything (false state) or include the full commit messages(true
state) from all related submodule changes. Add a third state called
SUBJECT_ONLY that only include the subjects of the commits from the
change history. Also set the default to be SUBJECT_ONLY.
Change-Id: I6baa16180c31f36ed3d61930834470c3c27387ed
c877ca9f5e