git-debrebase: Provide new get_tree and trees_diff_walk

git-debrebase: Provide new get_tree and trees_diff_walk

These are ways to avoid calling git-diff-tree, which is very slow even
with --name-only --no-renames and without recursion.  I think it is
doing all the work to make nice actual diffs, and then throwing them
away.

Two calls to git-ls-tree is about 20x faster than git-diff-tree
on the Linux kernel, for example.

No functional change in this commit, since no callers yet.

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