abandon/start: 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 ~30sec to ~3sec with the default -j8.

Change-Id: I0dc62d704c022dd02cac0bd67fe79224f4e34095
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297484


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