Not requiring a third Tab to show the list of possible completions

Not requiring a third Tab to show the list of possible completions
when the first Tab added the part that all matches have in common.

So now two Tabs in a row will always show the list of possible
completions -- if there /are/ any completions.  Which means that
a second Tab will either: 1) do nothing, when the name is complete
and exists; 2) beep, when nothing in the current directory starts
with the current string; 3) show the list of matches.

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@5656 35c25a1d-7b9e-4130-9fde-d3aeb78583b8