Fixed memory leaks by proper disposing references

I have tried to import a TFS repository containing ~23.000 changesets which in total has a size of ~17 GB.  Unfortunately the import always stopped with an "out of memory" exception (even though I have 8 GB RAM). So I used the .NET Memory Profiler from Scitech for finding the problems.

With the included changes the import works fine and doesn't consume more than 270 MB of memory, so the import "streams" propertly the way it should. From this change both the clone and fetch operations are affected.

Kind Regards,
Oliver