If the number of completions is greater than this, don't cycle.
This variable is a compromise between the traditional Emacs style of
completion, and the "cycling" style. Basically, if there are more
than this number of completions possible, don't automatically pick the
first one and then expect the user to press TAB to cycle through them.
Typically, when there are a large number of completion possibilities,
the user wants to see them in a list buffer so that they can know what
options are available. But if the list is small, it means the user
has already entered enough input to disambiguate most of the
possibilities, and therefore they are probably most interested in
cycling through the candidates. Set this value to nil if you want cycling to always be enabled.