strbuf: accept a comment string for strbuf_add_commented_lines()

strbuf: accept a comment string for strbuf_add_commented_lines()

As part of our transition to multi-byte comment characters, let's take a
NUL-terminated string pointer for strbuf_add_commented_lines() rather
than a single character.

All of the callers have to be adjusted; most can just pass
comment_line_str rather than comment_line_char.

And now our "cheat" in strbuf_commented_addf() can go away, as we can
take the full string from it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>