Fix search on Python 3

The Search dialog did not work under Python 3.  This is due to the fact that the result of calling the builtin function `map()` was being passed to the `QListWidget.addItems()` method in the `Search.set_commit_list()` method.  This worked fine under Python 2 because `map()` returns a list. Under Python 3 however it returns an iterator, which causes PyQt to raise an exception.  Fix by replacing the call to `map()` in the `Search.display_results()` method with a list comprehension.