Just checkout default master when workspace specified commit is not there.

[davidmlw]

There are cases that wit is stopped to compose a workspace when there is some commit is not there. It is strictly right. But the users may be frustrated at the point.

It is better still ckecking out the repo with a default, 'master', branch commit, and report this behavior to the users. So the users can have an non-perfect but composed workspace and then debug which repo is wrong, or still do his programming work if the wrong repo is not his/her current focus.