i18n: do not poison translations unless GIT_GETTEXT_POISON envvar is set

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>