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>
diff --git a/sequencer.c b/sequencer.c
index 6a1b7b2..852c3f9 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -667,11 +667,11 @@ void append_conflicts_hint(struct index_state *istate,
 	}
 
 	strbuf_addch(msgbuf, '\n');
-	strbuf_commented_addf(msgbuf, comment_line_char, "Conflicts:\n");
+	strbuf_commented_addf(msgbuf, comment_line_str, "Conflicts:\n");
 	for (i = 0; i < istate->cache_nr;) {
 		const struct cache_entry *ce = istate->cache[i++];
 		if (ce_stage(ce)) {
-			strbuf_commented_addf(msgbuf, comment_line_char,
+			strbuf_commented_addf(msgbuf, comment_line_str,
 					      "\t%s\n", ce->name);
 			while (i < istate->cache_nr &&
 			       !strcmp(ce->name, istate->cache[i]->name))