Do completion using `sh-shell-variables', but initialize it first.
This function is designed for use as the "completion table",
so it takes three arguments:
STRING, the current buffer contents;
PREDICATE, the predicate for filtering possible matches;
CODE, which says what kind of things to do.
CODE can be nil, t or `lambda'.
nil means to return the best completion of STRING, or nil if there is none.
t means to return a list of all possible completions of STRING.
`lambda' means to return t if STRING is a valid completion as it stands.