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.

This reverts commit 53d94b8091b36847bb9e495c76bb5a3ec2a2fdb5.

The reason for the revert is that the commit in question introduced a
regression where certain modules, functions and classes that were
exposed before were no longer exposed.

See https://github.com/gitpython-developers/GitPython/pull/1352#issuecomment-932757204
for additional information.

All `from <module> import *` has now been replaced by
`from <module> import X, Y, ...`.

Contributes to #1349