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.
87beb76dcc