Counting the number of days worked for all commiters to a git repo
For this part:

author_lines = filter(lambda str: str.startswith('Author'), lines)
date_lines = filter(lambda str: str.startswith('Date'), lines)
author_lines = map(lambda str: str[8:], author_lines)
date_lines = map(lambda str: str[8:18].strip(), date_lines)


This might be clearer, not that I have anything against map or filter, but list comprehensions do combine them nicely when you need both:

author_lines = [line[8:] for line in lines if line.startswith('Author')]
date_lines = [line[8:18].strip() for line in lines if line.startswith('Date')]


This:

lines = sorted(list(set(lines)), key = lambda tup: tup[0])


Can become:

lines = sorted(set(lines), key = lambda tup: tup[0])


for slightly less repetition (sorted automatically converts to a list).
And are you sure the key is even necessary? Tuples get sorted by elements just fine, the only reason to sort specifically by only the first element is if you want to preserve the original order of lines with the same author, rather than sorting them by the date line.
... Actually, why are you even sorting this at all? I don't see anything in the rest of the code that will work any differently whether it's sorted or not.

For this:

commiters = list(set(map(lambda tup: tup[0], lines)))


Why are you zipping author_lines and date_lines and then unzipping again? Just do:

commiters = set(author_lines)


or am I missing something?

And this:

d = dict(zip(commiters, [0 for x in xrange(len(commiters))]))
for item in lines:
    d[item[0]] += 1


You're just getting commit counts, right?  Use Counter:

import collections
d = collections.Counter([author_line for author_line,_ in lines])


Or, if your python version doesn't have collections.Counter:

import collections
d = collections.defaultdict(lambda: 0)
for author_line,_ in lines:
    d[author_line] += 1


... Wait, are you even using date_lines anywhere? If not, what are they there for?