gitlog-to-changelog: trim only trailing whitespaces

gitlog-to-changelog: trim only trailing whitespaces

This is fix for --format regression introduced by commit
2b93079a5d1baa4d;  it caused that --format='%s%n%n%b%n' (see the
doubled %n string) had no effect anymore.  This format
specification has been used e.g. by GNU paxuitils (commit
edfd8bcc3).

* build-aux/gitlog-to-changelog (main): Stop squashing multiple
newlines in commmit messages.