Improve custom conflict resolution usability

• The lines in the blue area are displayed shorter than the ones in the green/red areas  (see both screenshots below)
  • Expected: The blue area should have the same line length as the green and red areas.
  • Reason: This would help compare the content between areas

• The blue area shows four lines that are before those in the green one which are the one important, in "current version" option

• • Problem: It's hard to understand what will be the end result. Also, it's impossible to display the whole context before (limited to three lines) so the first one in the blue area is displayed nowhere else.
  • Suggestion 1: Don't include the lines before
  • Suggestion 2: Include the context lines before and after but darken their color to catch the eye on the important ones.

• In the case of only lines added, but the option selected is "latest version": The blue area shows a lot of lines that are before those in the green one and two that are after.

• • Problem: It's hard to understand what will be the end result. Also, it's impossible to display the whole context (limited to three lines before and nothing after the lines in the green area).
  • Suggestion: Display a text like "Nothing changes"

For context, this was displayed when updating an extension which has been customized before. I was comparing the "New file" with the "Merged file". Maybe this tool shouldn't be used in this context, it doesn't make a lot of sense: it's not a 3-way merge because the "Merged file" is already the result of the 3-way merge.