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