gettext tests: add GETTEXT_POISON=YesPlease Makefile parameter

gettext tests: add GETTEXT_POISON=YesPlease Makefile parameter

Add a new GETTEXT_POISON=YesPlease Makefile debug target. With this
target every invocation of _(s) will return "# GETTEXT POISON #"
instead of `s' or the msgstr that `s` would look up.

This target is intended to sanity check the gettextization series
against the test suite.

If the test suite is relying on the US English output of Git the test
should either have a NO_GETTEXT_POISON prerequisite, or the message in
question shouldn't be translated because it's a plumbing message.

We won't catch everything with this since we don't have 100% test
coverage, but the chances of us doing so are much larger.

The reason I'm using "# GETTEXT POISON #" is that a lot of tests that
rely on the fake editor would break if it wasn't a comment, since the
comment message that's inserted on e.g. `git commit --amend' is
translatable.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>