Add tips to delete local branches that has been squash and merged in the remote

[lmarvaud]

Very usefull when commit policy of the organisation is to squash en merge or rebase and merge the pull requests.

As a complement to git fetch -p :

git fetch -p && git branch -vv | grep ': gone]' | awk '{print $1}' | xargs git branch -D

source : stackoverflow: Remove tracking branches no longer on remote

Maybe should we also put the git fetch -p && part in git-tips ?! (not sure..)