commit-graph: expire commit-graph files

commit-graph: expire commit-graph files

As we merge commit-graph files in a commit-graph chain, we should clean
up the files that are no longer used.

This change introduces an 'expiry_window' value to the context, which is
always zero (for now). We then check the modified time of each
graph-{hash}.graph file in the $OBJDIR/info/commit-graphs folder and
unlink the files that are older than the expiry_window.

Since this is always zero, this immediately clears all unused graph
files. We will update the value to match a config setting in a future
change.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>