Update our copy of Dominik Reichl's SHA1 implementation

The upstream for our implementation of SHA1 has a version 2.1; we have no documentation of what version we've got; the diff between our sha1.cpp and the upstream 2.1 version is a single hunk changing most of the lines of the file. It is not clear how much of that is our adaptation of the upstream code to C++ and our conventions, but clearly there have been substantive changes. We need to evaluate whether an update would be an improvement; if it is, it would be worth documenting how we transform the upstream to adapt it to our needs, to make future update-checks more transparent. In particular, we should record (in qt_attribution.json) the version used.