Log repo.* config variables in git trace2 logger.

Bug: [google internal] b/181758736
Testing:
- Unit tests
- Verified repo git trace2 logs had expected data

Change-Id: I9af8a574377bd91115f085808c1271e9dee16a36
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299182


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