Fix git-cherry-pick.rst example.

[herrerog]

Using the parent of the cherry-picked commit as ancestor is not correct sometimes. That looks to be an issue because base was not used at all in the example whereas it should be used as ancestor.

[herrerog]

Sorry I got that one wrong actually @jdavid
We should use cherry.parents[0].tree as ancestor otherwise the resulting merge may be wrong.
Here what needs to be used:

repo = pygit2.Repository('/path/to/repo')

cherry = repo.revparse_single('9e044d03c')
basket = repo.branches.get('basket')

base_tree = cherry.parents[0].tree

index = repo.merge_trees(base_tree, basket, cherry)
tree_id = index.write_tree(repo)

author    = cherry.author
committer = pygit2.Signature('Archimedes', '[email protected]')

repo.create_commit(basket.name, author, committer, cherry.message,
                   tree_id, [basket.target])
del None # outdated, prevent from accidentally using it

Let me know if I should send another pull request.

[jdavid]

@herrerog Yes please