Comment (by Roland Haas):

pdftex actually does have a pipe option: https://tex.stackexchange.com/a/20566 which would significantly simplify things:

\begin{document}

\begingroup
\catcode`\%=11 \input|"git log --format=%ad --date=format:%Y-%m-%d -n 1 HEAD"
\endgroup

% foo bar

\end{document}

but this syntax is also not known to htlatex (and also requires the -enable-write18 command line option)

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2330/include-date-of-build-and-git-hash-in