These aliases seem to be common, see #7389 and others. This prevents
recursion on that example, so `alias ssh "env TERM=screen ssh"` will just
have the same completions as ssh.
Checking the last token is a heuristic which hopefully works for most
cases. Users are encouraged to use functions instead of aliases.
The description for an alias which already has escape sequences will
use backslash escapes for quoting; usually `string escape` can simply
quote it. Use a regex that accepts either escaping style.
This did some weird unescaping to try to extract the first word.
So we're now more likely to be *correct*, and the alias benchmark is
about 20% *faster*.
Call it a win-win.
