Handle forks in the `branch-history` GitHub CI job

[neilmayhew]

https://github.com/IntersectMBO/cardano-ledger/actions/runs/9843781035/job/27183341050?pr=4478

Run git fetch origin master amw/fix_allocation
  git fetch origin master amw/fix_allocation
  if git log --merges --oneline --decorate origin/master..origin/amw/fix_allocation | grep .; then
    echo 'Branch must not contain merges'
    false
  fi
  shell: /usr/bin/bash -e {0}
fatal: couldn't find remote ref amw/fix_allocation
Error: Process completed with exit code 128.