rerere: write out each record of MERGE_RR in one go

Instead of writing the hash for a conflict, a HT, and the path
with three separate write_in_full() calls, format them into a
single record into a strbuf and write it out in one go.

As a more recent "rerere remaining" codepath abuses the .util field
of the merge_rr data to store a sentinel token, make sure that
codepath does not call into this function (of course, "remaining" is
a read-only operation and currently does not call it).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.c

@@ -65,16 +65,18 @@ static int write_rr(struct string_list *rr, int out_fd)
 {
 	int i;
 	for (i = 0; i < rr->nr; i++) {
-		const char *path;
-		int length;
+		struct strbuf buf = STRBUF_INIT;
+
+		assert(rr->items[i].util != RERERE_RESOLVED);
 		if (!rr->items[i].util)
 			continue;
-		path = rr->items[i].string;
-		length = strlen(path) + 1;
-		if (write_in_full(out_fd, rr->items[i].util, 40) != 40 ||
-		    write_str_in_full(out_fd, "\t") != 1 ||
-		    write_in_full(out_fd, path, length) != length)
+		strbuf_addf(&buf, "%s\t%s%c",
+			    (char *)rr->items[i].util,
+			    rr->items[i].string, 0);
+		if (write_in_full(out_fd, buf.buf, buf.len) != buf.len)
 			die("unable to write rerere record");
+
+		strbuf_release(&buf);
 	}
 	if (commit_lock_file(&write_lock) != 0)
 		die("unable to write rerere record");