If non-nil, expand input command history references on completion.
This mirrors the optional behavior of tcsh (its autoexpand and histlit).
If the value is `input', then the expansion is seen on input.
If the value is `history', then the expansion is only when inserting
into the buffer's input ring. See also `comint-magic-space' and
`comint-dynamic-complete-functions'.
This variable supplies a default for `comint-input-autoexpand',
for Shell mode only.