strbuf: accept a comment string for strbuf_stripspace()

strbuf: accept a comment string for strbuf_stripspace()

As part of our transition to multi-byte comment characters, let's take a
NUL-terminated string pointer for strbuf_stripspace(), rather than a
single character. We can continue to support its feature of ignoring
comments by accepting a NULL pointer (as opposed to the current behavior
of a NUL byte).

All of the callers have to be adjusted, but they can all just pass
comment_line_str (or NULL).

Inside the function we detect comments by comparing the first byte of a
line to the comment character. We'll adjust that to use starts_with(),
which will match multiple bytes (though for now, of course, we still
only allow a single byte, so it's academic).

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