BUG: Fixed extension creation in version-controlled repository

It was only possible to create extension in a non-version-controlled directory.
The problem was that extension was created in repositoryRoot/extensionName directory
but when an extension is opened in a version-controlled directory then the extension
is expected to be found in repositoryRoot.

Changed extension creation so that if it is created in a version-controlled directory
then files are added to repositoryRoot (instead of repositoryRoot/extensionName).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24452 3bd1e089-480b-0410-8dfb-8563597acbee