This is a long-standing issue with how `complete --do-complete` does
its argument parsing: It takes an optional argument, so it has to be
attached to the token like `complete --do-complete=foo` or (worse)
`complete -Cfoo`.
But since `complete` doesn't take any bare arguments otherwise (it
would error with "too many arguments" if you did `complete -C foo`) we
can just take one free argument as the argument to `--do-complete`.
It's more of a command than an option anyway, since it entirely
changes what the `complete` call _does_.
This enables some limited use of arguments for wrapping completions. The
simplest example is that complete gco -w 'git checkout' now works like
you would want: `gco <tab>` now invokes git's completions with the
`checkout` argument prepended.
Fixes#1976
