command: add a helper for the parallel execution boilerplate

Now that we have a bunch of subcommands doing parallel execution, a
common pattern arises that we can factor out for most of them.  We
leave forall alone as it's a bit too complicated atm to cut over.

Change-Id: I3617a4f7c66142bcd1ab030cb4cca698a65010ac
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/301942


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