Open workspace should be able to check out new branch of git source

It's confusing that you can't change an existing workspace when you track a different branch. For example:

• I do bst workspace open on my element with a git source
• I decide I need to build a different branch so I change the branch in the element file and do bst track
• My workspace doesn't update its branch, and doesn't contain the commit on the new branch.
• I have to close, delete, and reopen the workspace

It would be nice if bst track updated the open workspaces.