i18n: git-grep "--open-files-in-pager" message

Gettextize the "--open-files-in-pager only works on the worktree"
message. A test in t7811-grep-open.sh explicitly checked for this
message. Change it to skip under GETTEXT_POISON=YesPlease.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -1061,7 +1061,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	}
 
 	if (show_in_pager && (cached || list.nr))
-		die("--open-files-in-pager only works on the worktree");
+		die(_("--open-files-in-pager only works on the worktree"));
 
 	if (show_in_pager && opt.pattern_list && !opt.pattern_list->next) {
 		const char *pager = path_list.items[0].string;

t/t7811-grep-open.sh

@@ -61,7 +61,7 @@ test_expect_success SIMPLEPAGER 'git grep -O' '
 	test_cmp empty out
 '
 
-test_expect_success 'git grep -O --cached' '
+test_expect_success C_LOCALE_OUTPUT 'git grep -O --cached' '
 	test_must_fail git grep --cached -O GREP_PATTERN >out 2>msg &&
 	grep open-files-in-pager msg
 '