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>

add-patch.c

@@ -1139,7 +1139,7 @@ static int edit_hunk_manually(struct add_p_state *s, struct hunk *hunk)
 	for (i = 0; i < s->buf.len; ) {
 		size_t next = find_next_line(&s->buf, i);
 
-		if (s->buf.buf[i] != comment_line_char)
+		if (!starts_with(s->buf.buf + i, comment_line_str))
 			strbuf_add(&s->plain, s->buf.buf + i, next - i);
 		i = next;
 	}

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);

trailer.c

@@ -1013,7 +1013,7 @@ static void parse_trailers(struct trailer_info *info,
 	for (i = 0; i < info->trailer_nr; i++) {
 		int separator_pos;
 		char *trailer = info->trailers[i];
-		if (trailer[0] == comment_line_char)
+		if (starts_with(trailer, comment_line_str))
 			continue;
 		separator_pos = find_separator(trailer, separators);
 		if (separator_pos >= 1) {

wt-status.c

@@ -1382,7 +1382,7 @@ static int read_rebase_todolist(const char *fname, struct string_list *lines)
 			  git_path("%s", fname));
 	}
 	while (!strbuf_getline_lf(&line, f)) {
-		if (line.len && line.buf[0] == comment_line_char)
+		if (starts_with(line.buf, comment_line_str))
 			continue;
 		strbuf_trim(&line);
 		if (!line.len)