repo: add trace support to the launcher

Now that we have a central run_command point, we can easily add
tracing support to the launcher script.

Change-Id: I9e0335c196cafd6263ff501925abfe835f036c5e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254755
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>