Since Git's branch.enablerebase config option only works on branches created after enabling the setting, this iterates over your projects and their branches to enable it for existing branches.

enablerebase.sh

#!/bin/bash

# Enables automatic rebase when pulling for each existing branch in your
# projects. This is useful in conjunction with the 'branch.enablerebase always'
# option, which enables this setting on new branches but leaves existing
# branches intact.
#
# Pass this a directory which contains all of your projects and it will enable
# thie option for all existing local branches in each Git repo within it.

function main() {
  if [ -z $1 ]; then
    # print usage if not passed a workspace
    usage
  else
    enable_rebase_in_workspace $1
  fi
}

function usage() {
  echo
  echo "enablerebase PROJECTS_WORKSPACE"
  echo
  echo "Loops through all git repos in PROJECTS_WORKSPACE and enables rebase on all local branches"
  echo
  echo "Ex:"
  echo "enablerebase ~/workspace"
}

function enable_rebase_in_workspace() {
  WORKSPACE=$1
  echo working in $WORKSPACE
  echo

  ls $WORKSPACE | while read project; do
    enable_rebase_in_project $project
    echo
  done
}

function enable_rebase_in_project() {
  PROJECT=$1
  echo working with $WORKSPACE/$PROJECT
  cd $WORKSPACE/$PROJECT

  if [ -d $WORKSPACE/$PROJECT/.git ]; then
    local_branches | while read branch; do
      enable_rebase_for_branch $branch
    done
  else
    echo $PROJECT is not a git project, moving along
  fi
}

function enable_rebase_for_branch() {
  BRANCH=$1
  echo looking at $BRANCH of $PROJECT
  git config --local branch.$BRANCH.rebase true
}


function local_branches() {
  # strip out the leading whitespace and "* " on current branch
  git branch | cut -c 3-
}

main $*