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
itself.

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