Do not update the ref hot bit when checking isIndexLoaded

Do not update the ref hot bit when checking isIndexLoaded

DfsPackFile.isIndexLoaded() uses the DfsBlockCache.Ref.get() method
to check if the index loaded. However, using the get() method marks
a hot bit in the cache, which can cause the index to never be unloaded
and seem hotter than it really is. Add a has() method which only
checks if the value is not null and does not update the hot bit.

Change-Id: I7e9ed216f6e273e8f5d79ae573973197654419b4