checkout: add --jobs support

Use multiprocessing to run in parallel.  When operating on multiple
projects, this can speed things up.  Across 1000 repos, it goes from
~9sec to ~5sec with the default -j8.

Change-Id: Ida6dd565db78ff7bac0ecb25d2805e8a1bf78048
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297982


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