test: add PRMX test commit

This is a commit that is designed to test the PRMX integration between
GitHub and the PostgreSQL Hacker's email list.  It's a fork of the
GitGitGadget code used by the Git team for the same purpose, thanks
open source friends!  We appreciate what you've built and we're happy
to be able to reuse it.

For readability. It was a slight modularity violation to have fields in
PGShmemHeader that were only used by the allocator code in shmem.c. And
it was inconsistent that ShmemLock was nevertheless not stored there.
Moving all the allocator-related fields to a separate struct makes it
more consistent and modular, and removes the need to allocate and pass
ShmemLock separately via BackendParameters.

Merge InitShmemAccess() and InitShmemAllocation() into a single
function that initializes the struct when called from postmaster, and
when called from backends in EXEC_BACKEND mode, re-establishes the
global variables. That's similar to all the *ShmemInit() functions that
we have.

Reported-by: Greg Burd <greg@burd.me>
Diagnosed-by: Greg Burd <greg@burd.me>
Reviewed-by: Greg Burd <greg@burd.me>
Author: Greg Burd <greg@burd.me>
Signed-off-by: Greg Burd <greg@burd.me>
Discussion: https://postgr.es/m/db6fdb35a8665ad3c18be01181d44b31@postgrespro.ru
Backpatch-through: 14
Signed-off-by: Greg Burd <gregburd@amazon.com>

Author: gburd

README.md

@@ -19,3 +19,5 @@ about building PostgreSQL from the source code can be found at
 The latest version of this software, and related software, may be
 obtained at <https://www.postgresql.org/download/>.  For more information
 look at our web site located at <https://www.postgresql.org/>.
+
+test.