BUG: Fix extension index push corner case

Fix error pushing a user update branch to the extension index if the
branch name already exists but has not been fetched locally. This can
happen e.g. if an index update request was previously created from a
different clone of the extension index.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23001 3bd1e089-480b-0410-8dfb-8563597acbee