xdiff: don't attempt to use fuzzer inputs larger than 100k

This is the recommended approach from [0], and limiting the input was
suggested in https://github.com/google/oss-fuzz/issues/2076 when
discussing our broken coverage build.

0: https://github.com/google/oss-fuzz/blob/master/docs/new_project_guide.md#custom-libfuzzer-options-for-clusterfuzz

Differential Revision: https://phab.mercurial-scm.org/D5525

contrib/fuzz/xdiff.cc

@@ -22,6 +22,11 b' int hunk_consumer(long a1, long a2, long'
 
 int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size)
 {
+	// Don't allow fuzzer inputs larger than 100k, since we'll just bog
+	// down and not accomplish much.
+	if (Size > 100000) {
+		return 0;
+	}
 	auto maybe_inputs = SplitInputs(Data, Size);
 	if (!maybe_inputs) {
 		return 0;