Defer evaluation of modified until after it's set

With the reorg r348175, we now look at modified before it is
set. Rearrange things so that we can set include_metadata to either
yes, no or if-modified. This should fix the -R flag that was broken in
r348175, which broke WITH_REPRODUCIBLE_BUILD for kernels.

Feedback From: emaste@
Differential Revision: https://reviews.freebsd.org/D20480