This fixes tilde completion when doing for instance `ls ~<TAB>'. See
also: Alioth #312613.
Added _tilde() helper function.
Contrary to _expand() and __expand_tilde_by_ref(), _tilde() also leaves the
tilde (~) intact. Replacement of the tilde has been a complaint in other
bugreports, especially since plain bash completion (without bash-completion
installed) doesn't expand the tilde when doing a `ls ~<TAB>'.
To run the tests:
./run completion/ls.exp
./run unit/_tilde.exp
ccbf141e13