forall: move nested func out to the class

This is in preparation for simplifying the jobs support.  The nested
function is referenced in the options object which can't be pickled,
so pull it out into a static method instead.

Change-Id: I01d3c4eaabcb8b8775ddf22312a6e142c84cb77d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298722


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>