git_export_autodoc now detects non-documentation commits

This speeds up extraction significantly
Doc: Do a full scan in a few more cases.

This should be essentially like the old extractor, but still
retaining most of the increase in speed.
Doc: Improved robustness of extractor change detector.

Use git diff instead of git show.

This means that meerges now will be handled correctly.
Doc: Reduced verbosity.