Make completion for debugindex<tab><tab> show debugindexdot, too. The special handling for commands with names that are substrings of other commands (e.g. with st and strip) wasn't used with debug commands before.

#!/bin/sh

hg init

echo "Is there an error message when trying to diff non-existing files?"
hg diff not found

echo "Is there an error message when trying to add non-existing files?"
hg add not found