sync: add a "finished" message

Some people find the existing output to be a bit confusing.  It spews
a lot of git output before exiting, but it's not exactly clear what
the final state is when things pass.  Add an explicit message.

Bug: https://crbug.com/gerrit/10501
Change-Id: I9de83b595d3185feb820005b8fc81c6adc55b357
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254732
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>