HistoryCompleter: Fix current text() handling

Can be seen in the Debugger's "Command:" input: Selecting a
history entry with "Return" properly executes the selected
item, but put something else into the line edit.

Change-Id: I2efa05374d9c31e8a80219794f2dbaaf50a01f9b
Reviewed-by: Christian Stenger <christian.stenger@digia.com>