Set a default branch name when creating a branch

[jamesaoverton]

When the user clicks "Create branch" we should suggest a default branch name.

GitHub does it like this: "jamesaoverton-patch-2". It increments the numeric part until it finds a name that's not used. I'm tempted to do it this way. It will require an API call, but that's probably a good idea anyway. Let's try this first.

If this doesn't work. The alternative is to use a date or timestamp or random string.