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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_git_changeset():
 """Get git identifier; taken from Django project."""

    git_log = Popen(
        'git log --pretty=format:%ct --quiet -1 HEAD',
        stdout=PIPE, stderr=PIPE, shell=True, universal_newlines=True)
    timestamp = git_log.communicate()[0]
    try:
        timestamp = datetime.utcfromtimestamp(int(timestamp))
    except ValueError:
        return None
    return timestamp.strftime('%Y%m%d%H%M%S')