Delete all remote branches on your fork `mine` which have already been merged into upstream `origin/master`

Local Branches

$ git branch  --merged master | 
  grep -v '*' | 
  grep -v master |
  xargs -L1 | 
  xargs git branch -d

Local repository

$ git branch -r --merged origin/master | 
  grep mine | 
  grep -v '>' | 
  grep -v master | 
  xargs -L1 | 
  xargs git branch -r -d

Remote repository

$ git branch -r --merged origin/master | 
  grep mine | 
  grep -v '>' | 
  grep -v master | 
  xargs -L1 | 
  awk '{split($0,a,"/"); print a[2]}' | 
  xargs git push mine --delete