Fixed #22867 -- Memoized django.utils.version.get_git_changeset().

This follows commits 80f4487 and 01399fa; original patch had to be
reverted because it wasn't Python 2.6 compatible and we need it to
be in order to build docs on the djangoproject.com server.

This fix should be replaced by @lru_cache as soon as we drop
Python 2.6 compatibility.

Thanks Florian Apolloner for the review and Alexander Schepanovski
for the original patch.

django/utils/version.py

@@ -57,6 +57,10 @@ def get_git_changeset():
     This value isn't guaranteed to be unique, but collisions are very unlikely,
     so it's sufficient for generating the development version numbers.
     """
+    # FIXME: Replace with @lru_cache when we upgrade the docs server to PY2.7+.
+    if hasattr(get_git_changeset, 'cache'):
+        return get_git_changeset.cache
+
     repo_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
     git_log = subprocess.Popen('git log --pretty=format:%ct --quiet -1 HEAD',
             stdout=subprocess.PIPE, stderr=subprocess.PIPE,
@@ -65,5 +69,9 @@ def get_git_changeset():
     try:
         timestamp = datetime.datetime.utcfromtimestamp(int(timestamp))
     except ValueError:
-        return None
-    return timestamp.strftime('%Y%m%d%H%M%S')
+        changeset = None
+    else:
+        changeset = timestamp.strftime('%Y%m%d%H%M%S')
+
+    get_git_changeset.cache = changeset
+    return changeset