storage/filesystem: dotgit, add a test for reading refs from a directory
storage/filesystem: dotgit, add a test for reading refs from a directory

Specifically, the problem demonstrated is that on some OS (e.g. FreeBSD),
read() on a directory will succeed.  The current implementation will accept
that and return incorrect results, rather than rejecting the directory.

go-git/go-git,go-git/go-git