sync: add separate --jobs options for different steps

The number of jobs one wants to run against the network tends to
factor differently from the number of jobs one wants to run when
checking out local projects.  The former is constrained by your
internet connection & server limits while the later is constrained
by your local computer's CPU & storage I/O.  People with beefier
computers probably want to keep the network/server jobs bounded a
bit lower than the local/checkout jobs.

Change-Id: Ia27ab682c62c09d244a8a1427b1c65acf0116c1c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/302804


Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>