Allow selecting the git revision to be packaged by "make dist".

Allow selecting the git revision to be packaged by "make dist".

Commit 619bc23a1 changed "make dist" to invoke "git archive",
but hard-wired the call to specify that the HEAD revision should
be packaged.  Our tarball building process needs to be able to
specify which git commit to package (notably, for packaging
back branches).  While we could make that work with some hackery
to operate in detached-HEAD state, it's a lot nicer just to expose
git archive's ability to specify what to package.  Hence, invent
a new make variable PG_GIT_REVISION.  This is undocumented, but
so is "make dist".

Also make corresponding changes in the meson scripts.  We have no
near-term intention of using that for package building, but it
will likely happen eventually, so stay prepared.

Discussion: https://postgr.es/m/3552543.1713909947@sss.pgh.pa.us