Run from a git repo to get stats on how many pull request merges could have been fast-forwarded.

fast_forward_check.sh

#!/usr/bin/env bash
set -ue

merge_shas="`git rev-list HEAD --merges --grep='^Merge pull request #[0-9]\{1,\} '`"
merges="`echo "$merge_shas" | wc -l | tr -d ' '`"
fast_forwards=0
non_fast_forwards=0

while read -r merge_sha; do
  first_parent="`git rev-list -1 $merge_sha^1`"
  second_parent="`git rev-list -1 $merge_sha^2`"
  merge_base="`git merge-base $first_parent $second_parent`"

  if [[ $merge_base = $first_parent || $merge_base == $second_parent ]];
  then
    ((fast_forwards++))
  else
    ((non_fast_forwards++))
  fi
done <<< "$merge_shas"

echo "Out of $merges merged pull requests, $fast_forwards were fast-forwardable and $non_fast_forwards were not."