This allows eg, putting .git/annex/tmp on a ram disk, if the disk IO
of temp object files is too annoying (and if you don't want to keep
partially transferred objects across reboots).
.git/annex/misctmp must be on the same filesystem as the git work tree,
since files are moved to there in a way that will not work cross-device,
as well as symlinked into there.
I first wanted to put the tmp objects in .git/annex/objects/tmp, but
that would pose transition problems on upgrade when partially transferred
objects existed.
git annex info does not currently show the size of .git/annex/misctemp,
since it should stay small. It would also be ok to make something clean it
out, periodically.
a1432bce2f