diff-tree: teach single-commit diff-tree to honour grafts
diff-tree: teach single-commit diff-tree to honour grafts

We used to read the commit objects by hand and ignored the grafts.
Rewrite it using lookup_commit() API, to make it grafts-aware.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

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