Renaming: Add .sh extension to completion script

[inkarkat]

This doesn't matter if (as currently recommended) the script is placed into a eagerly loaded location (like /etc/bash_completion.d/) - any name will do.
However, there's now lazy loading of completion scripts (in /usr/share/bash-completion/completions/), and that only works when the completion script is named exactly like the command the completion is for. As our command is todo.sh (ignoring aliases, which become more complex with lazy loading), the corresponding completion needs to be todo.sh (with the .sh extension) as well. Renaming does not do any harm for our recommended location, but makes it easier for users (and packagers who prepare a todo.sh package) that want to use lazy loading.

See #383 for the complete discussion.

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