Don't assume that the remote is always origin and that the remote branch name is the same as the local one.

For example, I can have a branch named `foo` that follows the branch `bar` in the remote `github`.
Without this patch `git-delete-branch foo` will try to delete `origin/foo` when it should really delete `github/bar`.

If the branch has no config it uses the default values, origin as remote and the same branch name.