Patch Set 1: I would prefer that you didn't submit this; IP review completed

Hmm... From a usability point of view, I would opt against this behavior. The user will not understand why a branch name is suggested sometimes and somtimes not. They might even start to type in the name of the "local"-ized remote branch name just to see the same error message. I think they are better off seeing the message immediately and get a chance to overwrite the name before continuing.
It might be worthwile, however, to think about selecting the complete name (no matter whether an error is shown) so that the first key typed (other than "End") overwrites the complete text, i.e. the user does not have to mark the text first if they need to enter a completely new name.