Introduce core.trustLooseRefStat config

Introduce core.trustLooseRefStat config

With repositories on NFS, JGit can read an old value of a loose ref
or miss the existence of a loose ref if file attributes of the loose
ref or its parent directories are cached by NFS. Introduce a new config
'core.trustLooseRefStat' that will optionally refresh file attributes of
the loose ref (at least on some NFS clients).

Possible values for this new config are:

* always: Trust loose ref file attributes (default)
* after_open: Similar to 'always', but refresh the file attributes of
              the loose ref and its parent directories before trusting
              it

The default is set to always trust the file attributes as after_open is
known to degrade performance.

In a subsequent change, SnapshottingRefDirectory will be updated to
cache the directories that were refreshed to avoid duplicate work and
thereby improve performance to some extent for the after_open setting.

Change-Id: I9dfaeaf5307b2b51ce6ee4bfd9e0678786685fcf
Signed-off-by: Kaushik Lingarkar <quic_kaushikl@quicinc.com>