Extract pack directory last modified check code Pulling the last modified checking logic out of ObjectDirectory makes it possible to reuse this code for other files, such as the $GIT_DIR/config or $GIT_DIR/packed-refs files. Change-Id: If2f27a89fc3b7adde7e65ff40bbca5d55b98b772 Signed-off-by: Shawn O. Pearce <spearce@spearce.org>