Determine hard-linking and nlink support per FileStore

Determine hard-linking and nlink support per FileStore

It's quite possible that JGit can use the hard-linking mechanism
for atomic file creation on some volumes but not on others.
Ultimately it depends on the file systems on the mounted volumes.

Cache the information per FileStore instead of using a single
global flag. Also catch FileSystemException, it may be thrown
if the operating system reports a failure. The previously caught
AccessDeniedException is a sub-class of FileSystemException.

Bug: 547332
Change-Id: I1ef672b3468b0be79e71674344f16f28f9d11ba1
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>