diff-tree: clean up diff_tree_stdin() function
diff-tree: clean up diff_tree_stdin() function

Split it into the "one commit" vs "two trees" case, since we'll
want to use the "one commit" case for other things too.

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