In cases where a file gets renamed such that the directories containing
it previous and after the rename have a common prefix, then git will
avoid printing this prefix twice and instead format the rename as
"prefix/{old => new}". We currently didn't do anything like that, but
simply printed "prefix/old -> prefix/new".

Adjust our behaviour to instead match upstream. Adjust the test for this
behaviour to expect the new format.

Until now, we didn't have any tests that verified that our format for
renames in subdirectories is correct. While our current behaviour is no
different than for renames that do not happen with a common prefix
shared between old and new file name, we intend to change the format to
instead match the format that upstream git uses.

Add a test case for this to document our current behaviour and to show
how the next commit will change that format.

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.