xdiff: move sign comparison warning guard into each file

xdiff: move sign comparison warning guard into each file

Allow each file to fix the warnings guarded by the macro separately by
moving the definition from the shared xinclude.h into each file that
needs it.

xmerge.c and xprepare.c do not contain any signed vs. unsigned
comparisons so the definition was not included in these files.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>