svn: Optimize propget performance after checkout

The goal here is to recursively get svn:externals property values
from a freshly checked out working copy at a given revision.

But passing a non null revision parameter to proget will make it get
the property values from the remote repository regardless of the checked
out revison and can lead to really long execution times due to a high
amount of network requests.

So force to get the property values from the local working copy
by setting the revision parameter to None.

Related to T611