[js2]

In my PATH I have this script as git-gsr, which I can call as "git gsr".

    #!/bin/sh
    
    usage () {
        cat >&2 <<'__USAGE__'
    usage: git gsr [-P | --perl-regexp] <old> <new> [paths...]
    
      replace all occurrances of <old> with <new> optionally limited to
      <paths...> (as interpreted by git grep)
    
      -P, --perl-regexp     interpret <old> as perl regular expression;
                            default is to treat it as a fixed string.
    
    __USAGE__
        exit 1
    }
    
    pattern='-F'
    perl='BEGIN {($old, $new) = (shift, shift)} s/\Q$old\E/$new/g'
    
    case "$1" in
        -P|--perl-regexp)
            shift
            pattern='-P'
            perl='BEGIN {($old, $new) = (shift, shift)} s/$old/$new/g'
            ;;
        -*) usage
            ;;
    esac
    test $# -lt 2 && usage
    old=$1; new=$2; shift; shift
    git grep -l -z $pattern "$old" -- "$@" |
    xargs -0 perl -pi -e "$perl" "$old" "$new"