bash-util/scripts/hashwalker

#!/usr/bin/env bash

. $(dirname $0)/util/logging.bash

: '
  *  Have a derivative project loosely based around another (à la copy-paste)
  *  that you would like to graft back onto the tree where it came from??
  *  Try hashwalker!!
  '

read -d '' usage <<EOUSAGE
Usage:  $0 BRANCH_A UPSTREAM_BRANCH [START_REFSPEC]

Note:  Commit hashes are walked along UPSTREAM_BRANCH.
EOUSAGE

[ -z "$1" -o -z "$2" ] && dedcat "$usage"

branch_a="$1"
upstream_branch="$2"
[ -n "$3" ] && refrange="$3~1..$upstream_branch" || refrange=$upstream_branch

hashes=( $(git log --format='%H' $refrange 2>/dev/null) )

declare -a smallest
for (( i=${#hashes[@]} - 1 ; i>=0 ; i-- )); do
  # Shrek was here.
  changed=$(($(echo $(git diff ${hashes[i]} $branch_a --shortstat \
                      | grep -o '[0-9]\+\s\+' | tail -n +2) \
               | sed 's/\s\+/+/g')))
  if [ ${#smallest[@]} -eq 0 ]; then
    smallest=( ${hashes[i]} $changed )
    echo -n 'f'
  else
    [ $changed -lt ${smallest[1]} ] && smallest[0]=${hashes[i]} && smallest[1]=$changed \
      && echo -n '!' || echo -n '.'
  fi
done
echo

echo "Smallest diff @ ${smallest[0]}: $(git diff ${smallest[0]} $branch_a --shortstat)"