Previously these intermediates were placed in the build directory but
immediately deleted. Unless we are going to make them easily available
for inspection (hopefully not often required), they should just go in
/tmp.
The automated changes to the output of git-format-patch looked okay, but
they were swallowing up binary changes. Now they are more conservative
and hopefully more robust. As an exception, hashes are still removed for
non-binary diffs.
For debugging, `scripts/flatten` now has a `--raw` option to skip this
post-processing step.
Soon I should try to stabilise the patch format.
