scatterd-dotfiles/bin/git-merge-branch

#!/bin/sh

set -e

if git symbolic-ref --short refs/remotes/origin/HEAD >/dev/null; then
  main_branch="$(git symbolic-ref --short refs/remotes/origin/HEAD | sed 's@^origin/@@')" "$@"
else
  echo "You don't have a primary branch reference set for your origin remote.
  Use:
  git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/{name_of_your_primary_branch}

  to set the reference and then try merging again."

  exit 1
fi

git fetch origin
line_count=$(git diff origin/$main_branch..$main_branch | wc -l)

if [ $line_count -gt 0 ]; then
  printf "failed: $main_branch is not up to date with origin/$main_branch\n"
  exit 1
fi

git checkout $main_branch
git merge "@{-1}"