prefer comment_line_str to comment_line_char for printing

prefer comment_line_str to comment_line_char for printing

As part of our transition to multi-byte comment characters, we should
use the string variable rather than the historical character variable.
All of the sites adjusted here are just swapping out "%c" for "%s" in
format strings, or strbuf_addch() for strbuf_addstr(). The type system
and printf-attribute give the compiler enough information to make sure
our formats and variable changes all match (especially important for
cases where the format string is defined far away from its use, like
prepare_to_commit() in commit.c).

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