We introduced a minor issue in #5757 that was causing the manpages to
be added every time we regenerate instead of replacing them. The
script was a bit unscrutable, and we do this block-replacement in
several places I thought it might be a good idea to have a dedicated
tool to do it.
This allows us to have simpler Makefiles whenever we update a
generated block inside another file. It's python, but does not have
dependencies :-)
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