strbuf: accept a comment string for strbuf_commented_addf()
As part of our transition to multi-byte comment characters, let's take a
NUL-terminated string pointer for strbuf_commented_addf() rather than a
single character.
All of the callers have to be adjusted, but they can just pass
comment_line_str rather than comment_line_char.
Note that we rely on strbuf_add_commented_lines() under the hood, so
we'll cheat a bit to squeeze our string into a single character (for now
the two are equivalent, and we'll address this TODO in the next patch).
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>