v2writable: fix epoch rollover on incremental imports
All of our internal epoch rollover calculations are done using
the estimated unpacked (and uncompressed) size of the repo. The
importer instance needs to check that unpacked size before
selecting an epoch when an epoch already has packed data.
This bug did not impact the initial mass imports since we only
initialize the Import instance once-per-epoch and did not need
to take existing epochs into account.
Tested manually with -mda on a local clone of LKML
Reported-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>