Send beeminder word count delta, not cumulative

beeminder.py

#!/usr/local/bin/python3
import re
import requests
import shlex
import subprocess

# login to beeminder and visit https://www.beeminder.com/api/v1/auth_token.json
auth_token = 'token'
username = 'username'
goal = 'thesis'

include = re.compile(r"(figures/|tables/|chapters/)?[^/]*.tex$")

url = 'https://www.beeminder.com/api/v1/users/{}/goals/{}/datapoints.json'.format(username, goal)

def count(commit):
    files = subprocess.check_output(["git", "ls-tree", "-r", "--name-only", commit]).decode('UTF-8').split()
    files = [fn for fn in files if include.match(fn)]

    cmd = "git show {}|texcount -|awk '/Words in text:/ {{print $4}}'".format(" ".join(commit + ":" + fn for fn in files))
    words = int(subprocess.check_output(cmd, shell=True))

    return words

previous = subprocess.check_output(shlex.split('git log HEAD~2..HEAD~1 --format="%H"')).strip().decode('UTF-8')
current = subprocess.check_output(shlex.split('git log HEAD~1..HEAD~0 --format="%H"')).strip().decode('UTF-8')
comment = subprocess.check_output(shlex.split("git log -1 --format=%B")).strip().decode('UTF-8')
delta = count(current) - count(previous)

if delta > 0:
    payload = {
        "value": delta,
        "comment": comment,
        "requestid": commit,
        "auth_token": auth_token
    }

    r = requests.post(url, data=payload)
    print('updated beeminder with {} words'.format(delta))