Upgrade xdiff to version used in core git 2.4.5 (0df0541).

Corrects an issue where an LF is added at EOF while applying
an unrelated change (ba31180), cleans up some unused code (be89977 and
e5b0662), and provides an improved callback to avoid leaking internal
(to xdiff) structures (467d348).

This also adds some additional functionality that we do not yet take
advantage of, namely the ability to ignore changes whose lines are
all blank (36617af).

This fixes all the warnings on win64 except those in deps, which
come from the regex code.

This cleans up the various GCC compiler warnings with the
xdiff code that was copied in.

This is the initial import of the xdiff code (LGPL) from
core git as of rev f349b562086e2b7595d8a977d2734ab2ef9e71ef