dch: workaround to make option '--git-author' work

git-dch option '--git-author' doesn't work when we want to use user.name and
user.email from git-config as changelog section trailer due to a possible bug
in dch.
This patch does a workaround on this issue.

Debian bug #740566 discussed about this.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=740566