fast-export: make sure updated refs get updated

fast-export: make sure updated refs get updated

When an object has already been exported (and thus is in the marks) it's
flagged as SHOWN, so it will not be exported again, even if in a later
time it's exported through a different ref.

We don't need the object to be exported again, but we want the ref
updated, which doesn't happen.

Since we can't know if a ref was exported or not, let's just assume that
if the commit was marked (flags & SHOWN), the user still wants the ref
updated.

IOW: If it's specified in the command line, it will get updated,
regardless of whether or not the object was marked.

So:

 % git branch test master
 % git fast-export $mark_flags master
 % git fast-export $mark_flags test

Would export 'test' properly.

Additionally, this fixes issues with remote helpers; now they can push
refs whose objects have already been exported, and a few other issues as
well. Update the tests accordingly.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>