[WARNING] please stop using git.git "next" for now

I just noticed there is a breakage in write-tree optimization
that uses the new cache-tree data structure in the "next"
branch.  Switching branches with "git checkout anotherbranch"
when your index exactly matches the current HEAD commit and then
immediately doing write-tree produces a nonsense tree, and
commits on top of that results in tree objects that have
duplicated entries.

I will be working on a fix now, but in the meantime please do
not use the "next" branch for real work.  Sorry for the
breakage.