Update to the xdiff used in git v2.35.0, with updates to our build
configuration to ignore the sort of warnings that we normally care
about (signed/unsigned mismatch, unused, etc.)

Any git-specific abstraction bits are now redefined for our use in
`git-xdiff.h`.  It is a (wildly optimistic) hope that we can use that
indirection layer to standardize on a shared xdiff implementation.