small update on the git checatsheet (wiki)

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1

Hi all,

it seems that rebasing is a confusing task for beginners (was for me too), so I updated the Git cheatsheet with small howtos on rebasing [1].

Best regards, Martin

[1] https://github.com/RIOT-OS/RIOT/wiki/Git-cheatsheet#how-to-rebase-your-master-on-a-current-riot-master