repo: show redirects when tracing commands

This copies the output style we use in git_command for showing output
and input redirections.

Change-Id: I449b27e7b262e1b30b24333109a1d91d9c7b1ce7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256453
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>