find multi-byte comment chars in NUL-terminated strings

Several parts of the code need to identify lines that begin with the
comment character, and do so with a simple byte equality check. As part
of the transition to handling multi-byte characters, we need to match
all of the bytes. For cases where we are looking in a NUL-terminated
string, we can just use starts_with(), which checks all of the
characters in comment_line_str.

Note that we can drop the "line.len" check in wt-status.c's
read_rebase_todolist(). The starts_with() function handles the case of
an empty haystack buffer (it will always return false for a non-empty
prefix).

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

sequencer.c

@@ -2003,7 +2003,7 @@ static int update_squash_messages(struct repository *r,
 			return error(_("could not read '%s'"),
 				rebase_path_squash_msg());
 
-		eol = buf.buf[0] != comment_line_char ?
+		eol = !starts_with(buf.buf, comment_line_str) ?
 			buf.buf : strchrnul(buf.buf, '\n');
 
 		strbuf_addf(&header, "%s ", comment_line_str);