sequencer: lib'ify checkout_fast_forward()

Instead of dying there, let the caller high up in the callchain
notice the error and handle it (by dying, still).

The only callers of checkout_fast_forward(), cmd_merge(),
pull_into_void(), cmd_pull() and sequencer's fast_forward_to(),
already check the return value and handle it appropriately. With this
step, we make it notice an error return from this function.

So this is a safe conversion to make checkout_fast_forward()
callable from new callers that want it not to die, without changing
the external behaviour of anything existing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge.c

@@ -57,7 +57,8 @@ int checkout_fast_forward(const unsigned char *head,
 
 	refresh_cache(REFRESH_QUIET);
 
-	hold_locked_index(lock_file, 1);
+	if (hold_locked_index(lock_file, 0) < 0)
+		return -1;
 
 	memset(&trees, 0, sizeof(trees));
 	memset(&opts, 0, sizeof(opts));
@@ -90,7 +91,9 @@ int checkout_fast_forward(const unsigned char *head,
 	}
 	if (unpack_trees(nr_trees, t, &opts))
 		return -1;
-	if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
-		die(_("unable to write new index file"));
+	if (write_locked_index(&the_index, lock_file, COMMIT_LOCK)) {
+		rollback_lock_file(lock_file);
+		return error(_("unable to write new index file"));
+	}
 	return 0;
 }