Bugs in co-authors computation in translation script

There's apparently some issues with the computation of co-authors:

• some authors appears twice
• some metadata information (e.g. cherry-pick info) appears as co-authors

See for example the produced commit: https://github.com/xwiki/xwiki-platform/commit/3118838d97b2997da8e08e1f4b6ad1d942fb1f71