support librepo.PackageTarget(endcb=...)

The main benefit of this is that we correctly display each file
as "done" only once. This solves the "headers ended" issues.

- Remove all "file end" checks from the progresscb code,
  as librepo now supports endcb parameter.

- Now 100% is regular update, but that's 3 chars instead of 2.
  Clamp this to 99% so we don't break the line layout.

- Update test cases.