forall: greatly speed up processing overhead

With the recent commit 0501b29e


("status: Use multiprocessing for `repo status -j<num>` instead of
threading"), the limitation with project serialization no longer
applies.  It turns out that ad-hoc logic is expensive.  In the CrOS
checkout (~1000 projects w/8 jobs by default), it adds about ~7sec
overhead to all invocations.  With a fast nop run:
	time repo forall -j8 -c true
This goes from ~11sec to ~4sec -- more than 50% speedup.

Change-Id: Ie6bcccd21eef20440692751b7ebd36c890d5bbcc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298724
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>