This utility function avoids an unnecessary update of the access time
for a loose object file.  Just as the atime isn't useful on a loose
object, its not useful on the pack or the corresonding idx file.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>