Like git annex runs git-annex, git-annex foo could run git-annex-foo when
it's not built-in.

One user of this would be annex-review-unused, which
its author would rather name git-annex-reviewunused if that
made "git annex reviewunused" work.

In CmdLine, where autocorrect is handled, it would need to 
search the path for all "git-annex-" commands and then 
either dispatch the one matching the inputcmdname,
or do autocorrect with the list of those commands 
included along with the builtins. --[[Joey]]