BUG: Include target in submit branch name

Include the target branch in the extension index branch created to
submit an extension. This is required to concurrently submit to multiple
target branches (which would otherwise be impossible due to branch name
collision).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22964 3bd1e089-480b-0410-8dfb-8563597acbee