Split colored conflict resolution to helper function, unify callers

Split colored conflict resolution to helper function, unify callers

- The cases in handleOverlappedFiles() and handleInstInstalledFile() only
  differ by presence of "other file state", unify the copy-paste code
  to helper function and comment a bit more.
- Push all the color fiddling into the helper, eliminating loads of
  unnecessary rpmfiFColorIndex() calls in the common case. Also gives
  us a single point of control for the color-conflict behavior.
- This is not supposed to change any behavior at all, just refactoring...