forall: allow interactive commands with -j1

Historically forall has been interactive since it ran in serial.
Recent rework in here dropped that to enable parallel processing.
Restore support for interactive commands when running -j1 or with
an explicit --interactive option.

Bug: https://crbug.com/gerrit/14256
Change-Id: I502007186f771914cfd7830846a4e1938b5e1f38
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/300722


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