Update _completion_shared.py

[bufferbit]

Wrap quotes around completion_path variable to solve potential issues from whitespace in the path name. I encountered this issue when using the --install-completion flag on a local module:

$ python3 -m cli --install-completion

This would create the file ~/.bash_completions/python3 -m cli.sh and broke code that referenced the unquoted path in .bashrc.

---

Something additional I thought of but could not figure out how to implement was adding the following code to .bashrc:

if [ -d "${HOME}/.bash_completions" ]; then
    for file in $(realpath -- "${HOME}/.bash_completions/*.sh"); do
        if [ -f "$file" ]; then
            source "$file"
        fi
    done
fi

This also solves the whitespace issue, but additionally removes the need to append a new line to .bashrc every time the --install-completion flag is used.