Fix NullReferenceException when browsing a subdirectory containing a submodule and .gitmodules cannot be read
Fix links for submodules in subdirectories

* .gitmodules is always read from the repository root
* submodules outside the current directory are ignored
Merge pull request #75 from jberezanski/submodules-in-subdirs

Fix browsing subdirectories containing submodules
Don't try to set the status code after streaming on an unbuffered connection.
This fixes #77.