proto: Deprecate `root_oid` field in the TreeEntry message
The TreeEntry message we're returning from the GetTreeEntries()
RPC
contains a root_oid
field. This field always points to the tree ID of
the commit via which GetTreeEntries()
arrived at the tree. This design
is quite weird, as the root OID would be the same for every single tree
entry returned by that RPC call. Furthermore, this field requires us to
spawn a separate Git process only to resolve that object ID and thus
slows down overall latency of the RPC.
Deprecate the field so that we can remove it in release 16.0. It is not known to be used by any clients anyway.
Part of #4823 (closed).