pkg/cover: simplify visual coverage comparison for two files

[tarasmadan]

No description provided.

[tarasmadan]

@dvyukov the steps I'm thinking about are:

1. Detect functions scope and for the equal functions remove the matching coverage markers.
2. onclick(fold functions). It'll fold the functions in both views.
3. Autofold functions if the coverage looks equal.
4. Parallel scrolling.
5. "Next diff" button.
6. Highlight the file diff.

[dvyukov]

Hard to comment, only dogfooding it for lots of cases can lead to an option that's not painful to use.
I am also thinking about higher-level story. Nobody will access all changes in these reports visually each week by clicking on all files. Other related functionalities: summary of changes, automated large change detection. These shouldn't be based on UI and lots of clicking. Solving these may also solve the problem for a single file (e.g. if summary points to functions with largest changes, it will be possible to search for the function name on the current view).