[amend] keep the amended message around in case of a conflict

[jerry-skydio]

if you're amending and you write out a nice amended message, but then hit a conflict reapply the rest of the stack, you will also lose your message which is inconvenient

revup should, in case of conflict, leave the message at some path and tell the user the message is still available. it only needs to do this if the message has actually changed vs what's in the commit