commit-graph: implement --set-latest

commit-graph: implement --set-latest

It is possible to have multiple commit graph files in a directory, but
only one is important at a time.

Use a 'graph-latest' file to point to the important file. Teach
git-commit-graph to write 'graph-latest' when given the "--set-latest"
option. Using this 'graph-latest' file is more robust than relying on
directory scanning and modified times.

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