forall: rewrite parallel logic

This fixes intermingling of parallel jobs and simplifies the code
by switching to subprocess.run.  This also provides stable output
in the order of projects by returning the output as a string that
the main loop outputs.

This drops support for interactive commands, but it's unclear if
anyone was relying on that, and the default behavior (-j2) made
that unreliable.  If it turns out someone still wants this, we can
look at readding it.

Change-Id: I7555b4e7a15aad336667292614f730fb7a90bd26
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297482


Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
Tested-by: Mike Frysinger <vapier@google.com>