Target branch selection for multi-file editor

The target branch isn't correctly selected by default and the target branch selection drop down needs fixing.

Proposal

• target branch should always default to the source branch
• targeting a different branch should switch you to the target branch

Creating a branch

Creating a branch should dismiss the drop down and switch the user to that branch.

Change target to a different existing branch when an open file is dirty (has edits)

Show warning that the user will lose unsaved changes.