Improve Merging UI

[ruifung]

Sometimes the tool mistakenly groups lines into one conflict area, even though they are in different sections, this results in not being able to view the contents of both sides as they are replaced with red/green blocks.

In my opinion, the option to merge only that line instead of the entire block should be an option, and an option to merge multiple blocks in one go by highlighting them would be useful too. Also, not blanking out the contents and displaying them with a red/green highlight would be a good idea too IMO.