This change seems like a good thing anyway for anyone who's tempted to use "make dist" manually, since they wouldn't necessarily want to package HEAD either. Now, if we just do it exactly like that then trying to "make dist" without setting PG_COMMIT_HASH will fail, since "git archive" has no default for its <tree-ish> argument. I can't quite decide if that's a good thing, or if we should hack the makefile a little further to allow PG_COMMIT_HASH to default to HEAD.
Just having it fail seems harsh. What if we had plain "make dist" at least output a friendly hint about "please specify a hash"? That seems better than an implicit HEAD default, as they can manually set it to HEAD themselves per the hint.
