We currently disallow creating a new change through CreateChange
endpoint against a non-existent branch, but we don't really have a
problem with that (except for preventing typos).
In this change, we split ChangeInfo into ChangeInfo and ChangeInput.
Added a new new_branch field in ChangeInput, default to false to
protect against typos, and when it's set to true, allow new change
against a non-existent branch.
Currently it's only implemented in the API, but not in the UI.
Change-Id: I6e661f036b121a33de183c126b763d29b5f2bc93
af6807f564