git-sh-i18n: check GETTEXT_POISON before USE_GETTEXT_SCHEME

git-sh-i18n: check GETTEXT_POISON before USE_GETTEXT_SCHEME

Running "make NO_GETTEXT=1 GETTEXT_POISON=1" currently fails
t0205.

While it might seem nonsensical at first glance to both
poison and disable gettext, it's useful to be able to do a
poison test-run on a system that doesn't have gettext at
all. And it works fine for C programs; the problem is only
with the shell code.

The issue is that we check the baked-in USE_GETTEXT_SCHEME
value before GETTEXT_POISON. And when NO_GETTEXT is set, the
Makefile sets USE_GETTEXT_SCHEME to "fallthrough".

So one fix would be to have the Makefile just set
USE_GETTEXT_SCHEME to "poison" if GETTEXT_POISON is set.
But there are two problems with that:

  1. USE_GETTEXT_SCHEME is actually a user-facing knob, so
     conceivably somebody could override it with:

       make USE_GETTEXT_SCHEME=gnu GETTEXT_POISON=1

     which would do the wrong thing (though that's much less
     likely than them having the variable set in their
     config.mak and just overriding GETTEXT_POISON on the
     command-line for a one-off test).

  2. We don't actually bake GETTEXT_POISON in to the shell
     library like we do for the C code. It checks
     $GIT_GETTEXT_POISON at runtime, which is set up by the
     test suite. So it makes sense to put the fix in the
     runtime code, too, which would cover something like:

       GIT_GETTEXT_POISON=foo git foo

     It's not likely that people use the poison code outside
     of running the test suite, but it's easy enough to make
     this case work.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>