Implement benchmark for looking up FileStore of a given Path

Implement benchmark for looking up FileStore of a given Path

Results on Mac OS 10.14.6 using APFS:

Result "org.eclipse.jgit.benchmarks.LookupFileStoreBenchmark.testLookupFileStore":
  44583.277 ±(99.9%) 2666.096 ns/op [Average]
  (min, avg, max) = (43707.073, 44583.277, 45320.685), stdev = 692.377
  CI (99.9%): [41917.181, 47249.373] (assumes normal distribution)

Secondary result
"org.eclipse.jgit.benchmarks.LookupFileStoreBenchmark.testLookupFileStore:·stack":
Stack profiler:

....[Thread state
distributions]........................................................
100.0%         RUNNABLE

....[Thread state:
RUNNABLE].............................................................
 42.5%  42.5% sun.nio.fs.UnixNativeDispatcher.realpath0
 37.9%  37.9% sun.nio.fs.UnixNativeDispatcher.stat0
 11.0%  11.0% sun.nio.fs.BsdNativeDispatcher.getfsstat
  7.6%   7.6% sun.nio.fs.BsdNativeDispatcher.fsstatEntry
  0.7%   0.7% sun.nio.fs.BsdNativeDispatcher.endfsstat
  0.1%   0.1% sun.nio.fs.UnixPath.initOffsets
  0.0%   0.0% sun.nio.fs.UnixFileAttributes.get
  0.0%   0.0% java.util.zip.Inflater.inflateBytes
  0.0%   0.0% sun.misc.Unsafe.compareAndSwapInt
  0.0%   0.0% sun.nio.fs.BsdFileStore.findMountEntry

This shows that FS.attrCacheByPath should be useful to reduce overhead
of looking up the FileStore of a given file.

Change-Id: I7213086b42e0453f0ee149660d507dac2a4644cc
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>