create a git symbolic-ref for REL9_3_STABLE

I suppose we'll be branching off 9.3 in a few weeks.  That event always
creates a service gap in the build farm and similar services, and a race
in the NLS service to get everything adjusted to the new branch.

It seems to me we could already now create a git symbolic-ref named
REL9_3_STABLE that points to master, get all those services updated for
the new name, and when the actual branching is supposed to happen, we
just remove the symbolic-ref and make a real branch, and everything else
should already be in place.

I have never actually used symbolic-ref, but after playing with it a
little bit, it seems it should work fine for this purpose.

Comments?