rerere: handle leftover rr-cache/$ID directory and postimage files

rerere: handle leftover rr-cache/$ID directory and postimage files

If by some accident there is only $GIT_DIR/rr-cache/$ID directory
existed, we wouldn't have recorded a preimage for a conflict that
is newly encountered, which would mean after a manual resolution,
we wouldn't have recorded it by storing the postimage, because the
logic used to be "if there is no rr-cache/$ID directory, then we are
the first so record the preimage".  Instead, record preimage if we
do not have one.

In addition, if there is only $GIT_DIR/rr-cache/$ID/postimage
without corresponding preimage, we would have tried to call into
merge() and punted.

These would have been a situation frustratingly hard to recover
from.

Signed-off-by: Junio C Hamano <gitster@pobox.com>