sync: fix recursive fetching

Commit b2fa30a2 ("sync: switch network
fetch to multiprocessing") accidentally changed the variable passed to
the 2nd fetch call from |missing| to |to_fetch| due to a copy & paste
of the earlier changed logic.  Undo that to fix git submodule fetching.

Bug: https://crbug.com/gerrit/14489
Change-Id: I627954f80fd2e80d9d5809b530aa6b0ef9260abb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305262


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