forall: simplify arg passing to worker children

The ProjectArgs function can be inlined which simplifies it quite a
bit.  We shouldn't need the custom exception handling here either.
This also makes the next commit easier to review.

Change-Id: If3be04f58c302c36a0f20b99de0f67e78beac141
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298723


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