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>

git_trace2_event_log.py

@@ -132,6 +132,18 @@ class EventLog(object):
     exit_event['code'] = result
     self._log.append(exit_event)
 
+  def CommandEvent(self, name, subcommands):
+    """Append a 'command' event to the current log.
+
+    Args:
+      name: Name of the primary command (ex: repo, git)
+      subcommands: List of the sub-commands (ex: version, init, sync)
+    """
+    command_event = self._CreateEventDict('command')
+    command_event['name'] = name
+    command_event['subcommands'] = subcommands
+    self._log.append(command_event)
+
   def DefParamRepoEvents(self, config):
     """Append a 'def_param' event for each repo.* config key to the current log.