Two issues there:
* Deleting branches:
I currently don't support this in fast-import, but I'll add support
for it. Its actually pretty simple to tell it to drop a branch,
especially if the dang thing doesn't actually exist in the git
repository yet (because its only in-memory).
* Creating a branch without refs/heads/ prefix:
This is a bug. I had good intentions by trying to verify the
name was one that didn't contain special reserved characters,
but I wound up also requiring you to create branches only in the
refs/heads/ namespace. That was not what I wanted to do. I'm
patching it tonight.