diff-tree: use new base_name_compare() helper function
diff-tree: use new base_name_compare() helper function

This fixes diff-tree sorting of directories vs files (we used to
use just the regular cache_name_compare() which only works on
full file pathnames).

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git