git-ffwd-update

#!/bin/bash

main() {
  REMOTES="$@";
  if [ -z "$REMOTES" ]; then
    REMOTES=$(git remote);
  fi
  REMOTES=$(echo "$REMOTES" | xargs -n1 echo)
  CLB=$(git branch -l|awk '/^\*/{print $2}');
  echo "$REMOTES" | while read REMOTE; do
    git remote update $REMOTE
    git remote show $REMOTE -n \
    | awk '/merges with remote/{print $5" "$1}' \
    | while read line; do
      RB=$(echo "$line"|cut -f1 -d" ");
      ARB="refs/remotes/$REMOTE/$RB";
      LB=$(echo "$line"|cut -f2 -d" ");
      ALB="refs/heads/$LB";
      NBEHIND=$(( $(git rev-list --count $ALB..$ARB 2>/dev/null) +0));
      NAHEAD=$(( $(git rev-list --count $ARB..$ALB 2>/dev/null) +0));
      if [ "$NBEHIND" -gt 0 ]; then
        if [ "$NAHEAD" -gt 0 ]; then
          echo " branch $LB is $NBEHIND commit(s) behind and $NAHEAD commit(s) ahead of $REMOTE/$RB. could not be fast-forwarded";
        elif [ "$LB" = "$CLB" ]; then
          echo " branch $LB was $NBEHIND commit(s) behind of $REMOTE/$RB. fast-forward merge";
          git merge -q $ARB;
        else
          echo " branch $LB was $NBEHIND commit(s) behind of $REMOTE/$RB. reseting local branch to remote";
          git branch -l -f $LB -t $ARB >/dev/null;
        fi
      fi
    done
  done
}

main $@

From this answer on SO. (By the same author.)