Bash script that can be used as a git alias to reword the commit message of an arbitrary commit on the current branch.

git-reword.sh

reword() {
    if [ -z "$1" ];
    then
        echo "No SHA provided. Usage: \"git reword <SHA>\"";
        exit 1;
    fi;
    if [ $(git rev-parse $1) == $(git rev-parse HEAD) ];
    then
        echo "$1 is the current commit on this branch.  Use \"git commit --amend\" to reword the current commit.";
        exit 1;
    fi;
    git merge-base --is-ancestor $1 HEAD;
    ANCESTRY=$?;
    if [ $ANCESTRY -eq 1 ];
    then
        echo "SHA is not an ancestor of HEAD.";
        exit 1;
    elif [ $ANCESTRY -eq 0 ];
    then
        git stash;
        START=$(git rev-parse --abbrev-ref HEAD);
        git branch savepoint;
        git reset --hard $1;
        git commit --amend;
        git rebase -p --onto $START $1 savepoint;
        git checkout $START;
        git merge savepoint;
        git branch -d savepoint;
        git stash pop;
    else
        exit 2;
    fi
};
reword