[xmodem]

How about

  #!/bin/sh
  BRANCHES_POINTING_TO_HEAD=$(git branch --contains HEAD  |wc -l)
  CURRENT_BRANCH=$(git branch --show-current)
  if [ "$BRANCHES_POINTING_TO_HEAD" -gt 1 ]; then
    if [ "$CURRENT_BRANCH" = "master" ]; then
      echo "You're committing to master even though other branches exist."
      echo "override with git commit  --no-verify"
      exit 1
    fi
  fi

You'll need git 2.22 for the --show-current git option.

But also I would suggest instead using

  git checkout -b <new_branch>

to create and switch branches in one command

[dheera]

A better UX would be

    $ git checkout <new_branch>
    <new_brach> doesn't exist. Would you like to create it (Y/n):

The real problem with "git" is just way too many options. For automated tools there can be a "-n" non-interactive option that fails with an error code and doesn't ask questions.