[Pkg-rust-maintainers] Bug#993216: dh-cargo timestamp fix doesn't cover changelogs installed to /usr/share/doc

[plugwash]

package: dh-cargo

Recently a substantial number of upstream cargo packages started using 
timestamps the ftpmasters
consider reject-worthy, I believe this was done in the name of 
reproducibility.

After it became clear that this was a larger-scale issue and we got sick 
of working around this in individual packages, sylvestre implemented a 
workaround in dh_cargo. Unfortunately this fix does not seem to be complete.

Specifically it seems that when an upstream changelog is installed by 
dh_installchangelogs  to /usr/share/doc it doesn't get the timestamp 
fixing treatment. Presumablly because dh_installchangelogs pulls it from 
the source tree while the dh_cargo timestamp fixing happens on the 
output tree.

I've worked around it for now in crossbeam-deque, but I don't know how 
many other packages are
potentially effected (e.g. have changelogs with dodgy timestamps in 
their upstream packaging) and whether this is something we need to deal 
with centrally.