lei_mirror: forkgroups use `git fetch --multiple'

lei_mirror: forkgroups use `git fetch --multiple'

This offloads network parallelization and safety off to git
itself while reducing the amount of unnecessary process spawning
we do.  This also improves readability of pack-refs invocations
and reduces the need for them.

To prevent heavily-forked repos from hitting system command-line
size limits, we group refs to be updated in the "fgrptmp" group.