multi-line prompt commands wider than terminal do not reposition to start on their own lines

fish moves long commands to start on their own line to help readability.
I use a multi-line prompt to make the first character of each command fall in the same column, to help readability. This conflicts with fish's feature, which changes the starting column of the command.
This patch changes the behavior of multi-line prompts to keep long commands in place.
![fish-rationale](https://f.cloud.github.com/assets/395175/1546098/d568c9aa-4d7f-11e3-8a09-e3c212f9c557.png)