The diff constructor functions had some confusing names, where the
"old" side of the diff was coming after the "new" side.  This
reverses the order in the function name to make it less confusing.

Specifically...

* git_diff_index_to_tree becomes git_diff_tree_to_index
* git_diff_workdir_to_index becomes git_diff_index_to_workdir
* git_diff_workdir_to_tree becomes git_diff_tree_to_workdir