Fix detection of copyright holders

* There is a copyright notice in `src/crypto/sha256_sse4.cpp` in the
  middle of the file which contains a comma before the copyright
  holder name. Correctly detect this so it's added to the report.
* Add missing copyright holders so that `copyright_header.py report`
  doesn't show any unexpected copyright holder names anymore.
* Exclude files from git subtrees because they are maintained at the
  original source.

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