Correct git-show recipe

Make the diff generation more idiomatic and fix the assembling of the
timestamp. git-show normally prints the author time, so use this instead
of the commit time. Also fix how tzinfo is obtained. Of course we have
to use the author's time zone and not some fixed one as I had written
before.

docs/recipes/git-show.rst

@@ -31,7 +31,7 @@ Show SHA hash
 Show diff
 ======================================================================
 
-    >>> diff = commit.parents[0].tree.diff_to_tree(commit.tree)
+    >>> diff = repo.diff(commit.parents[0], commit)
 
 ======================================================================
 Show all files in commit
@@ -41,14 +41,37 @@ Show all files in commit
     >>>     print(e.name)
 
 ======================================================================
-Produce something like a `git show` message
+Produce something like a ``git show`` message
 ======================================================================
 
+In order to display timezone information you have to create a subclass of
+tzinfo as described in the `Python datetime documentation`_::
+
+    from datetime import tzinfo, timedelta
+    class FixedOffset(tzinfo):
+        """Fixed offset in minutes east from UTC."""
+
+        def __init__(self, offset):
+            self.__offset = timedelta(minutes = offset)
+
+        def utcoffset(self, dt):
+            return self.__offset
+
+        def tzname(self, dt):
+            return None
+
+        def dst(self, dt):
+            return timedelta(0) # dealing with DST would be too complicated
+
+.. _Python datetime documentation: https://docs.python.org/2/library/datetime.html#tzinfo-objects
+
+Then you can make your message:
+
     >>> from __future__ import unicode_literals
     >>> from datetime import datetime
-    >>> import pytz
-    >>> tzinfo  = pytz.timezone('Europe/Berlin')
-    >>> dt      = datetime.fromtimestamp(float(commit.commit_time), tzinfo)
+    >>>
+    >>> tzinfo  = FixedOffset(commit.author.offset)
+    >>> dt      = datetime.fromtimestamp(float(commit.author.time), tzinfo)
     >>> timestr = dt.strftime('%c %z')
     >>> msg     = '\n'.join(['commit {}'.format(commit.tree_id.hex),
     ...                      'Author: {} <{}>'.format(commit.author.name, commit.author.email),