Fix bug that compare method was not restored to global value

This patch fixes the bug that compare method was not changed back to full contents after switching to quick contents. This happens when we switch for quick contents method to compare big files. Originally spotted and reported by Matthias Mayer in patch items #2802243 and further discussed in patch item #2802427.

The bug itself was misuse of global compare method variable as current file's compare method. The solution is to add new current compare method variable (per-file) which tells compare method for current file. Most of the time this is same than global (per compare) variable. It differs only when we temporarily change compare method.

Global compare method is now given in constructor and stored to constant variable so it cannot be changed during the compare.