strbuf: avoid static variables in strbuf_add_commented_lines()

In strbuf_add_commented_lines(), we have to convert the single-byte comment_line_char into a string to pass to add_lines(). We cache the created string using a static-local variable. But this makes the function non-reentrant, and it's doubtful that this provides any real performance benefit given that we know the string always contains a single character. So let's just create it from scratch each time, and to give the compiler the maximal opportunity to make it fast we'll ditch the over-complicated xsnprintf() and just assign directly into the array. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>