This fixes problems with missing function prototypes and 64-bit
data issues on Windows.

This changes the size data to uint32_t, fixes the array growth
logic to use a simple 1.5x multiplier, and uses a generic inline
function for growing the array to make the git_array_alloc API
feel more natural (i.e. it returns a pointer to the new item).

This adds real tests for user-configured diff drivers and in the
process found a bunch of bugs.

This implements the loading of regular expression pattern lists
for diff drivers that search for function context in that way.
This also changes the way that diff drivers update options and
interface with xdiff APIs to make them a little more flexible.

This is a significant reorganization of the diff code to break it
into a set of more clearly distinct files and to document the new
organization.  Hopefully this will make the diff code easier to
understand and to extend.

This adds a new `git_diff_driver` object that looks of diff driver
information from the attributes and the config so that things like
function content in diff headers can be provided.  The full driver
spec is not implemented in the commit - this is focused on the
reorganization of the code and putting the driver hooks in place.

This also removes a few #includes from src/repository.h that were
overbroad, but as a result required extra #includes in a variety
of places since including src/repository.h no longer results in
pulling in the whole world.