Honor trustFolderStats also when reading packed-refs

Honor trustFolderStats also when reading packed-refs

Then list of packed refs was cached in RefDirectory based on mtime of
the packed-refs file. This may fail on NFS when attributes are cached.
A cached mtime of the packed-refs file could cause JGit to trust the
cached content of this file and to overlook that the file is modified.

Honor the config option trustFolderStats and always read the packed-refs
content if the option is false. By default this option is set to true
and this fix is not active.

Change-Id: I2b65cfaa8f4aba2efbf8a5e865d3f09f927e2eec