sequencer: signal failed ff as an aborted, not a conflicted merge

`do_pick_commit` handles three situations if it is not fast-forwarding.
In order for `do_pick_commit` to identify the situation, it examines the
return value of the selected merge command.

1. return value 0 stands for a clean merge
2. 1 is passed in case of a failed merge due to conflict
3. any other return value means that the merge did not even start

So far, the sequencer returns 1 in case of a failed fast-forward, which
would mean "failed merge due to conflict". However, a fast-forward
either starts and succeeds or does not start at all. In particular, it
cannot fail in between like a merge with a dirty index due to conflicts.

In order to signal the three possible situations (not only success and
failure to complete) after a pick through porcelain commands such as
`cherry-pick`, exit with a return value that is neither 0 nor 1. 128 was
chosen in line with the other situations in which the sequencer
encounters an error. In such situations, the sequencer returns a
negative value and `cherry-pick` translates this into a call to `die`.
`die` then terminates the process with exit status 128.

Signed-off-by: Fabian Ruch <bafain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -278,7 +278,7 @@ static int fast_forward_to(const unsigned char *to, const unsigned char *from,
 
 	read_cache();
 	if (checkout_fast_forward(from, to, 1))
-		exit(1); /* the callee should have complained already */
+		exit(128); /* the callee should have complained already */
 	ref_lock = lock_any_ref_for_update("HEAD", unborn ? null_sha1 : from,
 					   0, NULL);
 	strbuf_addf(&sb, "%s: fast-forward", action_name(opts));