Makefile: only add gettext tests on XGETTEXT_INCLUDE_TESTS=YesPlease

Makefile: only add gettext tests on XGETTEXT_INCLUDE_TESTS=YesPlease

Change the "pot" target so that the xgettext(1) invocation no longer
include test files from t/t0200.

These should only be included in the Icelandic is.po file since Git's
own test suite uses them for regression tests, but nobody else should
translate them. Previously I indicated this with a TRANSLATORS comment
for all the strings:

    #. TRANSLATORS: This is a test. You don't need to translate it.
    #, c-format
    msgid "TEST: A C test string %s"
    msgstr ""

But if translators follow those instructions they'll never have 100%
translation coverage. It's much better to add a hidden option so that
is.po now has to use a special git.pot file for msgmerge(1):

    make pot XGETTEXT_INCLUDE_TESTS=YesPlease

But everything else can continue to use the documented:

    make pot

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