rerere: record resolution even if file is not in merge base

rerere: record resolution even if file is not in merge base

Two-file merges were rare enough that they were dropped outside of the
radar.  This fix is a trivial change to builtin-rerere.c::find_conflict().
It is still sane to insist that we do not do rerere for symlinks, and
require to have stages #2 and #3, but we can drop the requirement to have
stage #1. rerere does not use information from there anyway.

This fix is from Junio, together with two tests to verify that it works
as expected.

Acked-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>