add a --trace-python option

This can help debug issues by tracing all the repo python code with
the standard trace module.

Change-Id: Ibb7f4496ab6c7f9e130238ddf3a07c831952697a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/234833
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>