Now that Repository#raw_repository no longer sets the autocrlf option it
will also no longer raise any NoRepository errors since it doesn't
access Rugged any more. This also means that Repository#exists? can't
simply return the raw repository as this is no indication of whether or
not the repository actually exists (besides returning a non boolean is
weird in the first place).

To solve this problem Repository#exists? now properly checks if the
repository exists and returns true/false instead of a
Gitlab::Git::Repository or nil object.