Update .mailmap file

# Finding out duplicates by comparing email addresses:
git shortlog -sne |awk '{ print $NF }' |sort |uniq -d

# Finding out duplicates by comparing names:
git shortlog -sne |awk '{ NF--; $1=""; print }' |sort |uniq -d

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>