Add a diff marker in collapsed block if any modification has been made in that block

[ilmoralito]

I usually work with collapsed code blocks except for where I currently work. Thus

Add a general diff marker when a block of code is collapsed. For example, a line was deleted, when the code block collapses, a general diff marker is displayed in order to indicate to the user that there are modifications in this section.

[srbs]

Feel free to add an enhancement request in the official issue tracker: https://github.com/SublimeTextIssues/Core/issues