Rework diff rendering to allow putting the cursor into deleted text, soft-wrapping and scrolling deleted text correctly

[maxbrunsfeld]

Closes #12553

• Fix diff_hunk_before
• Fix failure to show deleted text when expanding hunk w/ cursor on second line of the hunk
• Failure to expand diff hunk below the cursor.
• Delete the whole file, and expand the diff. Backspace over the deleted hunk, panic!
• Go-to-line now counts the diff hunks, but it should not
• backspace at the beginning of a deleted hunk deletes too much text
• Indent guides are rendered incorrectly
• Fix randomized multi buffer tests

Maybe:

• Buffer search should include deleted text (in vim mode it turns out I use /x all the time to jump to the next x I can see).
• vim: should refuse to switch into insert mode if selection is fully within a diff.
• vim o command when cursor is on last line of deleted hunk.
• vim shift-o on first line of deleted hunk moves cursor but doesn't insert line
• enter at end of diff hunk inserts a new line but doesn't move cursor
• (shift-enter at start of diff hunk does nothing)
• Inserting a line just before an expanded hunk collapses it

Release Notes:

• Improved diff rendering, allowing you to navigate with your cursor inside of deleted text in diff hunks.