Repository#file_at: don't throw an exception on nonexistent path
Repository#file_at: don't throw an exception on nonexistent path

If the path doesn't exist, trying to access blob_data[:oid] would
throw a NoMethodError. Check whether the file exists and return nil if
it doesn't instead.

libgit2/rugged,Acidburn0zzz/rugged,saraid/rugged,libgit2/rugged,saraid/rugged,libgit2/rugged,Acidburn0zzz/rugged,tpickett66/rugged,spraints/rugged,Acidburn0zzz/rugged,tpickett66/rugged,saraid/rugged,spraints/rugged,tpickett66/rugged