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 #611
Migrated from D7077 (view on Phabricator)