Encode relation id as protobuf tag in content

Because we store all relations in effectively the same column, we
use a different protobuf field number wrapped around the individual
entity message.  This allows scanning programs that just examine
every row to be able to tell different relation's types apart and
correctly inspect the fields of each entity.

Change-Id: I26161a69ba78e97f0145db3d781ae60e96f66e0e
Signed-off-by: Shawn O. Pearce <sop@google.com>
3 files changed