sequencer (rebase -i): write out the final message

The shell script version of the interactive rebase has a very specific
final message. Teach the sequencer to print the same.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

sequencer.c

@@ -2126,6 +2126,9 @@ cleanup_head_ref:
 		}
 		apply_autostash(opts);
 
+		fprintf(stderr, "Successfully rebased and updated %s.\n",
+			head_ref.buf);
+
 		strbuf_release(&buf);
 		strbuf_release(&head_ref);
 	}