git: pass `id` attribute of `pygit2.Tree` object

Authored by sheehan.


git: pass id attribute of pygit2.Tree object

pygit2's Repository object expects an instance of Oid
to return the corresponding object, instead of the object

After this change and D9062, hg commit -i seems to work,
unless it hits a case folding assertion (ie trying to add
a file like README.md).

Differential Revision: https://phab.mercurial-scm.org/D9068