Bug 26621: Adjust .mailmap to reduce the number of invalid authors

This file is a nightmare, really. We should simplify it and order it correctly.
Letting it here for now, I'd like to run the gitstats command on it to double check the output.

Signed-off-by: Martin Renvoize <[email protected]>

Signed-off-by: Katrin Fischer <[email protected]>

JD: Amended patch: remove dup occurrences for Brendan

Signed-off-by: Jonathan Druart <[email protected]>