Fall back when move optimization is invalid

[gaoflow]

Summary

• make the existing optimization flag control move-detection bookkeeping
• validate optimized patches that contain move operations before returning them
• fall back to a conservative no-move diff when the optimized patch cannot be applied back to the source
• add the three Move-detection bug #179 reproductions as regression coverage

Tests

• python -m unittest tests.MakePatchTestCase.test_issue179
• python tests.py
• python -m compileall jsonpatch.py tests.py
• git diff --check

Closes #179