Git hash of a tree object, including uncommitted changes

hash_dir.sh

#!/bin/bash
function usage {
    cat << EOL
Return the GIT hash for the tree object under a specified directory. This is
effectively a unique hash of only the contents of the files in the directory
and does not depend on commit data.

Usage: $0 [path/to/directory]
EOL
    exit 1
}

# Check for correct arguments
if [[ "$#" -ne 1 ]] || [[ ! -d "$1" ]]; then
    usage
else
    # Check for any untracked files in directory
    UNTRACKED=$(git ls-files --others "$1")
    if [[ -n "$UNTRACKED" ]]; then
        NFILES=$(wc -l <<< "$UNTRACKED")
        echo "$NFILES untracked files were found under '$1', either add or clean them."
        exit 1
    fi

    # The revision to create the tree-hash for, either a detached stash commit,
    # or 'HEAD' if the working tree is clean.
    STASH=$(git stash create)
    REVISION=${STASH:-HEAD} # Defaults to 'HEAD' if the stash was empty (clean working tree)

    # Return the tree-hash for $REVISION under the directory argument passed
    git rev-parse --short "$REVISION":"$1"
fi

I think there is an issue with the script that is, git ls-files --others "$1" will also include files matched the patterns defined in .gitignore.