provide option to include use_this_module in diffs

[aekiss]

If use_this_module is false for all compared files it won't be shown in a diff, which can give the misleading impression that differences in the remaining variables matter.

Provide an option to show use_this_module whenever it is false in all files and there are other differences in the group.

Could also provide an option to hide everything in groups which have use_this_module false.

[aekiss]

Mostly implemented in commit 017df1d