sync: capture all git output by default

The default sync output should show a progress bar only for successful
commands, and the error output for any commands that fail.  Implement
that policy here.

Bug: https://crbug.com/gerrit/11293
Change-Id: I85716032201b6e2b45df876b07dd79cb2c1447a5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297905


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