builtin-grep: pass ignore case option to external grep

Don't just read the --ignore-case/-i option, pass the flag on to the
external grep program.

Signed-off-by: Robert Fitzsimons <robfitz@273k.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-grep.c

@@ -459,6 +459,8 @@ static int external_grep(struct grep_opt *opt, const char **paths, int cached)
 		push_arg("-n");
 	if (opt->regflags & REG_EXTENDED)
 		push_arg("-E");
+	if (opt->regflags & REG_ICASE)
+		push_arg("-i");
 	if (opt->word_regexp)
 		push_arg("-w");
 	if (opt->name_only)