index: Add missing 'tree' entry to diff_to_tree docstring argspec

It's been missing since the original argspec was added with 5ed9eb4
(Add documentation for conflicts and fixup Index, 2014-07-10).
docs/merge: default_signature is an attribute, not a method

Avoid:

  >>> repo.default_signature()
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: '_pygit2.Signature' object is not callable
Merge remote-tracking branch 'wking/default-signature-attribute'