Insert text for tab completion when there is only one completion.

[alandekok]

When the only choice is "foo", and the user hits , the input buffer should fill with "foo", instead of showing that "foo" is the only selection.

[gaborcsardi]

:+1:

[w32zhong]

This is not a good idea since user may find the only choice is actually not what he wants, so maybe hit tag again (or hit escape) to go back to the original edit buffer.