This takes the `--stat` and related example options in the example
diff.c program and converts them to use the `git_diff_get_stats`
API which nicely formats stats for you.
I went to add bar-graph scaling to the stats formatter and noticed
that the `git_diff_stats` structure was holding on to all of the
`git_patch` objects. Unfortunately, each of these objects keeps
the full text of the diff in memory, so this is very expensive. I
ended up modifying `git_diff_stats` to keep just the data that it
needs to keep and allowed it to release the patches. Then, I added
width scaling to the output on top of that.
In making the diff example program match 'git diff' output, I ended
up removing an newline from the sumamry output which I then had to
compensate for in the email formatting to match the expectations.
Lastly, I went through and refactored the tests to use a couple of
helper functions and reduce the overall amount of code there.
