Upgrade `git blame lts` to `git blame --porcelain`

See this review discussion https://github.com/timvink/mkdocs-git-authors-plugin/pull/12#discussion_r387658346 and this related [stackoverflow question](https://stackoverflow.com/questions/60523415/robustly-retrieve-sha-and-line-content-with-git-blame-python3) 

