progress: include execution time summary

We're already keeping tracking of the start time, so might as
well use it to display overall execution time for steps.

Change-Id: Ib4cf8b2b0dfcdf7b776a84295d59cc569971bdf5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298482


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>