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/src/testdir/dumps/Test_diff_05.dump b/src/testdir/dumps/Test_diff_05.dump
new file mode 100644
index 0000000..7a5d540
--- /dev/null
+++ b/src/testdir/dumps/Test_diff_05.dump
@@ -0,0 +1,20 @@
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