repo: Add a new "command" event type to git trace2 logging in repo.

Add a new "event": "command", which is emitted at when all command
arguments have been processed.

Additional fields:

"name": Name of the primary command (ex: repo, git)

"subcommands"': List of the sub-commands once command-line arguments
                are processed

Examples:

Command: repo --version

Event: {"event": "command", <common fields>,
        "name": "repo",
	"subcommands": ["version"]
	}

Bug: [google internal] b/178507266
Testing:
- Unit tests
- Verified repo git trace2 logs had expected data

Change-Id: I825bd0ecedee45135382461a4ba10f987f09aef3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/300343


Reviewed-by: Ian Kasprzak <iankaz@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>