convert logmsg_reencode to get_commit_buffer

Like the callsites in the previous commit, logmsg_reencode
already falls back to read_sha1_file when necessary.
However, I split its conversion out into its own commit
because it's a bit more complex.

We return either:

  1. The original commit->buffer

  2. A newly allocated buffer from read_sha1_file

  3. A reencoded buffer (based on either 1 or 2 above).

while trying to do as few extra reads/allocations as
possible. Callers currently free the result with
logmsg_free, but we can simplify this by pointing them
straight to unuse_commit_buffer. This is a slight layering
violation, in that we may be passing a buffer from (3).
However, since the end result is to free() anything except
(1), which is unlikely to change, and because this makes the
interface much simpler, it's a reasonable bending of the
rules.

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

sequencer.c

@@ -154,7 +154,7 @@ static int get_message(struct commit *commit, struct commit_message *out)
 static void free_message(struct commit *commit, struct commit_message *msg)
 {
 	free(msg->parent_label);
-	logmsg_free(msg->message, commit);
+	unuse_commit_buffer(commit, msg->message);
 }
 
 static void write_cherry_pick_head(struct commit *commit, const char *pseudoref)