The function generate_propconflict() lossily converts a four-way conflict into a
three-way representation: the 4->3 mapping depends on the values of the 4 sides,
but it shouldn't. See attached thread for details.
Two changes should be made:
1) the 4->3 mapping should be divorced from the values of the 4 sides. See
r1595522 for the 4->3 mapping property conflicts now use. (The property mapping
converts NULL to "" for stringification into the prej file; but in
svn_wc_conflict_description3_t, we should be able to keep NULLs as NULLs.)
2) svn_wc_conflict_description3_t should grow an additional file slot, in order
for API users to have access to all four sides of the conflict, not just three.