status: Use multiprocessing for `repo status -j<num>` instead of threading

This change increases the speed of the command with parallelization with
processes.  The parallelization with threads doesn't work well, and
increasing the number of jobs to many (8 threads ~) didn't increase the speed.
Possibly, the global interpreter lock of Python affects.

Bug: https://crbug.com/gerrit/12389
Change-Id: Icbe5df8ba037dd91422b96f4e43708068d7be924
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/279936
Tested-by: Kimiyuki Onaka <kimiyuki@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>