i18n: fix auto detection of gettext scheme for shell scripts

A new code added by ad17ea7 (add a Makefile switch to avoid gettext
translation in shell scripts, 2012-01-23) tried to optionally force
a gettext scheme to "fallthrough", but ended up forcing it to everybody.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-sh-i18n.sh

@@ -21,7 +21,7 @@ GIT_INTERNAL_GETTEXT_SH_SCHEME=fallthrough
 if test -n "@@USE_GETTEXT_SCHEME@@"
 then
 	GIT_INTERNAL_GETTEXT_SH_SCHEME="@@USE_GETTEXT_SCHEME@@"
-elif test -n "@@USE_FALLTHROUGH_GETTEXT_SCHEME@@$GIT_INTERNAL_GETTEXT_TEST_FALLBACKS"
+elif test -n "$GIT_INTERNAL_GETTEXT_TEST_FALLBACKS"
 then
 	: no probing necessary
 elif test -n "$GIT_GETTEXT_POISON"