commit-graph: implement --delete-expired

commit-graph: implement --delete-expired

Teach git-commit-graph to delete the .graph files that are siblings of a
newly-written graph file, except for the file referenced by 'graph-latest'
at the beginning of the process and the newly-written file. If we fail to
delete a graph file, only report a warning because another git process may
be using that file. In a multi-process environment, we expect the previoius
graph file to be used by a concurrent process, so we do not delete it to
avoid race conditions.

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