Prevent stale packed-refs file when Gitaly is running on top of NFS

Users of the Linux NFS client v4.0 (and even v4.1) may encounter stale file handles or stale data if git gc is running for a long time. A patch for this kernel bug is here: https://marc.info/?l=linux-nfs&m=153816500525563&w=2

This workaround calls stat() on the packed-refs file to force the kernel to do a directory entry validation on the inode.

Closes #1339 (closed)