git-debrebase: Rewrite get_differs in terms of trees_diff_walk

As noted, git-diff-tree is quite slow.  We can do much better, by
stepping across the two trees ourselves.

There should be no particularly significant change: the answers should
be the same for reasonable inputs.  Unreasonable inputs (particularly,
executable files etc.) may be handled better, even.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>