repo: export GIT_TRACE2_PARENT_SID

This helps with people tracing repo/git execution.  We use a similar
format to git, but a little simpler since we always initialize the
env var setting, and we want to avoid too much overhead.

Bug: https://crbug.com/gerrit/12314
Change-Id: I75675b6cc4c6f7c4f5e09f54128eba9456364d04
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254331
Reviewed-by: Josh Steadmon <steadmon@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>