Add git-cherry-pick recipes

Add the way that worked for me. Not sure if it is idiomatic. When doing
the convenience-mode cherry-pick, the repo remains in cherry-picking
mode afterwards. I've already added an issue for this.

docs/recipes.rst

@@ -17,6 +17,7 @@ Main porcelain commands
 .. toctree::
    :maxdepth: 1
 
+   git-cherry-pick (Apply the changes introduced by some existing commits.) <recipes/git-cherry-pick>
    git-init (Create an empty git repository or reinitialize an existing one.) <recipes/git-init>
    git-log (Show commit logs.) <recipes/git-log>
    git-show (Show various types of objects.) <recipes/git-show>

docs/recipes/git-cherry-pick.rst

@@ -0,0 +1,71 @@
+**********************************************************************
+git-cherry-pick
+**********************************************************************
+
+The convenient way to cherry-pick a commit is to use
+:py:meth:`.Repository.cherrypick()`. It is limited to cherry-picking with a
+working copy and on-disk index.
+
+.. code-block:: bash
+
+    $> cd /path/to/repo
+    $> git checkout basket
+    $> git cherry-pick 9e044d03c
+
+.. code-block:: python
+
+    repo = pygit2.Repository('/path/to/repo')
+    repo.checkout('basket')
+
+    cherry_id = pygit2.Oid('9e044d03c')
+    repo.cherrypick(cherry_id)
+
+    if repo.index.conflicts is None:
+        tree_id = repo.index.write_tree()
+
+        cherry    = repo.get(cherry_id)
+        committer = pygit2.Signature('Archimedes', 'archy@jpl-classics.org')
+
+        repo.create_commit(basket.name, cherry.author, committer,
+                           cherry.message, tree_id, [basket.target])
+        del basket # outdated, prevent from accidentally using it
+
+
+----------------------------------------------------------------------
+Cherry-picking a commit without a working copy
+----------------------------------------------------------------------
+
+This way of cherry-picking gives you more control over the process and works
+on bare repositories as well as repositories with a working copy.
+:py:meth:`~.Repository.merge_trees()` can also be used for other tasks, for
+example `three-argument rebases`_.
+
+.. _`three-argument rebases`: https://www.kernel.org/pub/software/scm/git/docs/git-rebase.html
+
+.. code-block:: python
+
+    repo = pygit2.Repository('/path/to/repo')
+
+    cherry = repo.revparse_single('9e044d03c')
+    basket = repo.lookup_branch('basket')
+
+    base      = repo.merge_base(cherry.oid, basket.target)
+    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', 'archy@jpl-classics.org')
+
+    repo.create_commit(basket.name, author, committer, cherry.message,
+                       tree_id, [basket.target])
+    del None # outdated, prevent from accidentally using it
+
+----------------------------------------------------------------------
+References
+----------------------------------------------------------------------
+
+- git-cherry-pick_.
+
+.. _git-cherry-pick: https://www.kernel.org/pub/software/scm/git/docs/git-cherry-pick.html