This also adds a test case for the completion of arguments following
multi-argument aliases, to cover the bug reported in issue #5377.
The default command is like a special kind of alias, one which is
expanded from zero tokens. Consequently, it also triggers the bug
#5377 on bash/zsh, even if the `default-command` is just a single token.
The fix is along the lines sketched out by the "TODO" comment. Bash and
Zsh don't behave identical, so the padding ([""]) still needs to be
applied (and removed) conditionally in a disciplined manner.
