prune: add --jobs support

Use multiprocessing to run in parallel.  When operating on multiple
projects, this can greatly speed things up.  Across 1000 repos, it
goes from ~10sec to ~4sec with the default -j8.

This only does a simple conversion over to get an easy speedup.  It
is currently written to collect all results before displaying them.
If we refactored this module more, we could have it display results
as they came in.

Change-Id: I5caf4ca51df0b7f078f0db104ae5232268482c1c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298643


Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
Tested-by: Mike Frysinger <vapier@google.com>