cache-tree: sort the subtree entries.

cache-tree: sort the subtree entries.

Not that this makes practical performance difference; the kernel tree
for example has 200 or so directories that have subdirectory, and the
largest ones have 57 of them (fs and drivers).  With a test to apply
600 patches with git-apply and git-write-tree, this did not make more
than one per-cent of a difference, but it is a good cleanup.

Signed-off-by: Junio C Hamano <junkio@cox.net>