Members, subgroups, and owner updates should only result in writing
the updated content back to disk if something was changed. For
members and subgroups, we should also save on assembling the commit
message footers.
The mechanisms of VersionedMetaData already ensured that no unnecessary
commits would be created. So, the previous behavior wasn't wrong. We
can just be a bit more efficient.
Change-Id: I31e663d028d467426709259861083fbc490948ac
47bcef7dc0