Modify Index.write_tree() to have more robust Repository detection

Currently, Index.write_tree() relies on either the caller passing in a
`repo=` arg or the underlying `git_index` being already bound to a
`git_repository`. This ignores the case where the caller does not pass
a `repo` argument to `Index.write_tree()` but the `Index._repo`
property is populated on the index.

This change modifies Index.write_tree() to use the passed-in `repo`
argument, falls back to using `Index._repo` and then assumes that
`git_index` must be bound to a `git_repository`. This change should make
Index.write_tree() a little more robust in the most common use-case.

pygit2/index.py

@@ -157,6 +157,9 @@ class Index(object):
         It returns the id of the resulting tree.
         """
         coid = ffi.new('git_oid *')
+
+        repo = repo or self._repo
+
         if repo:
             err = C.git_index_write_tree_to(coid, self._index, repo._repo)
         else: