i18n: do not poison translations unless GIT_GETTEXT_POISON envvar is set

Tweak the GETTEXT_POISON facility so it is activated at run time
instead of compile time.  If the GIT_GETTEXT_POISON environment
variable is set, _(msg) will result in gibberish as before; but if the
GIT_GETTEXT_POISON variable is not set, it will return the message for
human-readable output.  So the behavior of mistranslated and
untranslated git can be compared without rebuilding git in between.

For simplicity we always set the GIT_GETTEXT_POISON variable in tests.

This does not affect builds without the GETTEXT_POISON compile-time
option set, so non-i18n git will not be slowed down.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gettext.c

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) 2010 Ævar Arnfjörð Bjarmason
+ */
+
+#include "git-compat-util.h"
+#include "gettext.h"
+
+int use_gettext_poison(void)
+{
+	static int poison_requested = -1;
+	if (poison_requested == -1)
+		poison_requested = getenv("GIT_GETTEXT_POISON") ? 1 : 0;
+	return poison_requested;
+}