set the generated flag on svn nodes only for .gitignore files

Normal file copies and deletes aren't generated, even when we have to create them from a directory copy or delete.

Also, use non-anchored ignore rules to represent SVN's implicit ignore rules in .gitignore files. This means we only need to generate one .gitignore file in the root of very branch. svn:ignore props still trigger new .gitignore files as before, but they don't need to repeat the implicit rules.