Initial commits can't be staged, remain stuck in STAGING state

It seems that commits can't be staged correctly if there does not yet exist a destination branch (i.e. the commit being staged is an initial commit).

Symptoms observed on this change http://codereview.qt-project.org/13963 were:

• commit displays as STAGING
• staging-ls command fails:

$ ssh -p 29418 codereview.qt-project.org gerrit staging-ls -p qt/qtimageformats.git -b refs/staging/master -d master
fatal: No such branch: qt/qtimageformats,refs/staging/master

Note: I've since created an empty initial commit to unblock staging of changes for this repository.
The messages on that change from "Cannot cherry-pick an initial commit onto an existing branch" onwards occurred after this, and are unrelated to this bug.

This can be easily worked around by always creating an empty initial commit as the initial content of the branch.