tutorial: Fix 2-change-file.txt, add example of write_tree_diff usage.

docs/tutorial/2-change-file.txt

@@ -7,11 +7,23 @@ As seen in the introduction, it's about making a path in a tree point to a
 new blob. The old blob will remain to compute the diff. The tree is altered
 and the new commit'task is to point to this new version.
 
-In the following examples, we assume we still have the ``repo`` and ``tree``
-object from the previous chapter.
+In the following examples, we will need a ``repo`` and an initial commit
+(``c1``) similar to the one made in the previous chapter::
+
+  >>> from dulwich.repo import Repo
+  >>> from dulwich.objects import Blob, Tree
+  >>> repo = Repo.init("myrepo", mkdir=True)
+  >>> blob = Blob.from_string("My file content\n")
+  >>> tree = Tree()
+  >>> tree.add(0100644, "spam", blob.id)
+  >>> repo.object_store.add_object(blob)
+  >>> repo.object_store.add_object(tree)
+  >>> c1_id = repo.do_commit("Initial commit.",
+  ...     committer="JaneExample <jane@example.com>", tree=tree.id)
 
 Let's first build the blob::
 
+  >>> from dulwich.objects import Blob
   >>> spam = Blob.from_string("My new file content\n")
   >>> spam.id
   '16ee2682887a962f854ebd25a61db16ef4efe49f'
@@ -29,12 +41,14 @@ opportunity of changing its mode::
 
 Now let's record the change::
 
+  >>> from dulwich.objects import Commit
+  >>> from time import time
   >>> c2 = Commit()
   >>> c2.tree = tree.id
-  >>> c2.parents = [commit.id]
-  >>> c2.author = c2.committer = author
+  >>> c2.parents = [c1_id]
+  >>> c2.author = c2.committer = "John Doe <john@example.com>"
   >>> c2.commit_time = c2.author_time = int(time())
-  >>> c2.commit_timezone = c2.author_timezone = tz
+  >>> c2.commit_timezone = c2.author_timezone = 0
   >>> c2.encoding = "UTF-8"
   >>> c2.message = 'Changing "spam"'
 
@@ -44,18 +58,31 @@ may happen to have several parents after merging branches.
 
 Remain to record this whole new family::
 
-  >>> object_store.add_object(spam)
-  >>> object_store.add_object(tree)
-  >>> object_store.add_object(c2)
+  >>> repo.object_store.add_object(spam)
+  >>> repo.object_store.add_object(tree)
+  >>> repo.object_store.add_object(c2)
 
 You can already ask git to introspect this commit using ``git show`` and the
-value of ``commit.id`` as an argument. You'll see the difference will the
+value of ``c2.id`` as an argument. You'll see the difference will the
 previous blob recorded as "spam".
 
+The diff between the previous head and the new one can be printed using
+write_tree_diff::
+
+  >>> from dulwich.patch import write_tree_diff
+  >>> import sys
+  >>> write_tree_diff(sys.stdout, repo.object_store, repo[c1_id].tree, tree.id)
+  diff --git a/spam b/spam
+  index c55063a..16ee268 100644
+  --- a/spam
+  +++ b/spam
+  @@ -1,1 +1,1 @@
+  -My file content
+  +My new file content
+
 You won't see it using git log because the head is still the previous
 commit. It's easy to remedy::
 
   >>> repo.refs['refs/heads/master'] = c2.id
 
-Now all git tools will work as expected. Though don't forget that Dulwich is
-still open!
+Now all git tools will work as expected.

dulwich/tests/__init__.py

@@ -81,6 +81,7 @@ def test_suite():
     tutorial = [
         '0-introduction',
         '1-initial-commit',
+        '2-change-file',
         ]
     tutorial_files = ["../../docs/tutorial/%s.txt" % name for name in tutorial]
     def setup(test):