handle a missing "file" in the diff output

A hunk might start like this:

    @@ -4,6 +4,18 @@

This can easily happen for a change near the start of a file.

With this sort of input, generate-changelog will die:

      File "/home/tromey/bin/scripts/generate-changelog", line 121, in <module>
	main()
      File "/home/tromey/bin/scripts/generate-changelog", line 118, in main
	p.on_line(line)
      File "/home/tromey/bin/scripts/generate-changelog", line 82, in on_line
	scope = m.group(2).split(' ')
    AttributeError: 'NoneType' object has no attribute 'group'

The fix is to check whether the regexp matched and use the empty
string for the scope.