OK button does nothing when no text in "local branch name" entry, and gives no feedback as to why

[Alex Launi]

Which makes this screen a little bit confusing, but luckily this is a papercut style bug and there are two easy fixes I can think of.

1) ok button on select branch window should be disabled when no local name given
2) local name should autofill with upstream branch name

I think applying both of these is the optimal solution. When no branch is chosen, OK should be desensitized, but when a branch is chosen, that field should populate itself with the branch name from upstream and OK should then become clickable.