commit-graph: emit trace2 cmd_mode for each sub-command

commit-graph: emit trace2 cmd_mode for each sub-command

Emit trace2_cmd_mode() messages for each commit-graph
sub-command.

The commit graph commands were in flux when trace2 was
making it's way to git. Now that we have enough sub-commands
in commit-graph, we can label the various modes within them.
Distinguishing between read, write and verify is a great
start.

Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Acked-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>