Test with some postimages that actually grow/shrink from the
original, adding new lines or removing them. (Also do so without
context to ensure that we can add/remove from a non-zero part of
the line vector.)
Patches can now come from a variety of sources - either internally
generated (from diffing two commits) or as the results of parsing
some external data.
Handle the application of binary patches. Include tests that
produce a binary patch (an in-memory `git_patch` object),
then enusre that the patch applies correctly.
