Read patches directly from git-log

git-format-patch has some annoying properties, among others: it leaves
an empty file (no header either!) if the diff of a commit is empty.
Read directly from git-log to avoid these issues.  It should also be
faster.
Refactor main code into a separate function
Implement a format that does not contain the patches
Split compute_assignment into three parts

compute_assignment really does three things: finding a good assignment
via the matching algorithm, reassembling the result into a
good-for-presentation list of commit changes, and printing.  Separate
them.
Optimize detection of equal commits

Greedily assign commits with the same data (author, message and patch)
to each other, starting at the beginning.  This should greatly speed
up diffs of long series that were only changed slightly.
Merge branch 'short-format'

Conflicts:
	git-tbdiff.py