Move conflict handling into two steps: load the conflicts and
then apply the conflicts. This is more compatible with the
existing checkout implementation and makes progress reporting
more sane.
If a D/F conflict or rename 2->1 conflict occurs,
we write the file sides as filename~branchname. If
a file with that name already exists in the working
directory, write as filename~branchname_0 instead.
(Incrementing 0 until a unique filename is found.)
