Add GHTorrent, some https, notabug

This commit adds a few sentences in relation to the first known attempt to
store and make available git repository hosting ephemera (GHTorrent,
introduced to us by Roberto Di Cosmo). Since one of the two sponsors of
GHTorrent is Microsoft, both the ethics and practical aspects of this in
the context of reproducibility and scientific ethics as expressed by the
international scientific community are rather unclear, so a link to one of
the well-known lists of practical and ethical issues with Github is
included.

A minor fix is made in 'tex/src/appendix-existing-solutions.tex', since the
word 'data' is plural (singular is 'datum').

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