git-branch-ttl-stats

#!/bin/bash
set -euo pipefail

main() {
    merge-commits | branch-ttl
}

merge-commits() {
    git log --first-parent --format="%h %at %s" | grep Merge
}

branch-ttl() {
    local rows=0

    while read -r merge merge_ts subject; do
        if [[ $rows -eq 0 ]]; then
            emit-header
        fi

        emit-row "$merge" "$(( merge_ts - "$(earliest-commit-timestamp "$merge")" ))" "$subject"

        : $(( rows++ ))
    done
    echo "Found $rows merges" >&2
}

emit-header() {
    emit-row merge_commit branch_ttl merge_subject
}

emit-row() {
    printf '%s\t%s\t%s\n' "$1" "$2" "$3"
}

earliest-commit-timestamp() {
    local merge="$1"
    git log --format="%at" "$merge^2" "^$merge^1" | sort -n | head -n1
}

main "$@"