patch 8.1.0360: using an external diff program is slow and inflexible

Problem:    Using an external diff program is slow and inflexible.
Solution:   Include the xdiff library. (Christian Brabandt, closes #2732)
            Use it by default.
diff --git a/Filelist b/Filelist
index f488839..343a2bd 100644
--- a/Filelist
+++ b/Filelist
@@ -273,6 +273,20 @@
 		src/libvterm/t/92lp1640917.test \
 		src/libvterm/t/harness.c \
 		src/libvterm/t/run-test.pl \
+		src/xdiff/xdiff.h \
+		src/xdiff/xdiffi.c \
+		src/xdiff/xdiffi.h \
+		src/xdiff/xemit.c \
+		src/xdiff/xemit.h \
+		src/xdiff/xhistogram.c \
+		src/xdiff/xinclude.h \
+		src/xdiff/xmacros.h \
+		src/xdiff/xpatience.c \
+		src/xdiff/xprepare.c \
+		src/xdiff/xprepare.h \
+		src/xdiff/xtypes.h \
+		src/xdiff/xutils.c \
+		src/xdiff/xutils.h \
 
 
 # source files for Unix only