Used git log instead of git show for last commit's timestamp

The reason for this was that git show included the whole changeset in
the output, but only the UTC timestamp was needed. By using git log
it is possible to get just the timestamp. The whole changeset can be
large, and can cause unicode encoding errors.

django/utils/version.py

@@ -40,10 +40,10 @@ def get_git_changeset():
     so it's sufficient for generating the development version numbers.
     """
     repo_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
-    git_show = subprocess.Popen('git show --pretty=format:%ct --quiet HEAD',
+    git_log = subprocess.Popen('git log --pretty=format:%ct --quiet -1 HEAD',
             stdout=subprocess.PIPE, stderr=subprocess.PIPE,
             shell=True, cwd=repo_dir, universal_newlines=True)
-    timestamp = git_show.communicate()[0].partition('\n')[0]
+    timestamp = git_log.communicate()[0]
     try:
         timestamp = datetime.datetime.utcfromtimestamp(int(timestamp))
     except ValueError: