The whitespace handling and trailer selection isn't very trivial or good
documented. It therefore seemed easier and less error prone to just call
git to parse the message for the trailers section and remove superfluos
whitespaces.

With the command `git interpret-trailers` git provides a way to interact
with trailer lines in the commit messages that look similar to RFC 822
e-mail headers (see: https://git-scm.com/docs/git-interpret-trailers).
The new property returns those parsed trailer lines from the message as
dictionary.