try_rename_commits_on_merge.sh

#!/bin/sh
#inspired by HellBrick

function replace_commit_message {
    commit_msg=$1
    ticket=$2
    to_insert=" ( ticket #$ticket )"

    # hook should not insert ticket id if it is already inserted
    already_ticketed=`grep -c "$to_insert" $commit_msg_file`
    if [ $already_ticketed -eq 0 ]
    then

        first_line=$(echo "$commit_msg" | head -n1)

        text="${first_line}${to_insert}"

        lines_count=$(echo "$commit_msg" | wc -l)

        if [ $lines_count -gt 1 ]
        then
            lines_after_first=$(echo "$commit_msg" | tail -n +2)
            text="${text}
${lines_after_first}"
        fi

        echo "$text"

    fi
}

commit_msg_file=$1
commit_type=$2

if [ "$commit_type" = "merge" ]
then

    githead=$(env | grep GITHEAD)
    source="${githead##*=}"
    target=$(git symbolic-ref HEAD)

    ticket=`awk -F '/' '{ print $NF }' <<< "$source" | grep -P '^[0-9]+\$'`

    if [ ! -z "$ticket" ]
    then
        # func call against cat & ticket should be instead of 'lel'
        git filter-branch --msg-filter 'lel' $target..$source
    else
        echo 'no ticket id in the branch name' >&2
        exit 1
    fi
fi