grep: prefer builtin over external one when coloring results

As far as I know, not all grep programs support coloring, so we should
rely on builtin grep. If you want external grep, set
color.grep.external to empty string.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -825,6 +825,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	if (opt.color && !opt.color_external)
+		builtin_grep = 1;
 	if (!opt.pattern_list)
 		die("no pattern given.");
 	if ((opt.regflags != REG_NEWLINE) && opt.fixed)