Fix right panel background extending beyond the truncation symbol

In side-by-side mode, if `background_color_extends_to_terminal_width`
is set, the left panel color is extended via spaces, but the right
one via an ANSI sequence which instructs the terminal emulator to
fill the background color rightwards.

However due to the padding after the right panel the color will
extend beyond a possible truncation symbol:

"TEXT#AB→#", not "TEXT#AB→ " with "#" being the green background.

If any line is truncated, the entire block is now padded with spaces
(if only selected lines used spaces the line would show a jagged
right edge)

Add enums `BgFillWidth` and `BgFillMethod` to better distinguish
if the background should be filled, and if so how (ANSI or Spaces).

src/features/line_numbers.rs

@@ -582,7 +582,7 @@ pub mod tests {
         let mut lines = output.lines().skip(7);
         let (line_1, line_2) = (lines.next().unwrap(), lines.next().unwrap());
         assert_eq!(strip_ansi_codes(line_1), " 1  ⋮    │a = 1");
-        assert_eq!(strip_ansi_codes(line_2), " 2  ⋮    │b = 2");
+        assert_eq!(strip_ansi_codes(line_2), " 2  ⋮    │b = 23456");
     }
 
     #[test]
@@ -606,7 +606,7 @@ pub mod tests {
         let mut lines = output.lines().skip(7);
         let (line_1, line_2) = (lines.next().unwrap(), lines.next().unwrap());
         assert_eq!(strip_ansi_codes(line_1), "    ⋮ 1  │a = 1");
-        assert_eq!(strip_ansi_codes(line_2), "    ⋮ 2  │b = 2");
+        assert_eq!(strip_ansi_codes(line_2), "    ⋮ 2  │b = 234567");
     }
 
     #[test]
@@ -688,7 +688,7 @@ pub mod tests {
         let mut lines = output.lines().skip(5);
         let (line_1, line_2) = (lines.next().unwrap(), lines.next().unwrap());
         assert_eq!(strip_ansi_codes(line_1), " 1  ⋮    │-a = 1");
-        assert_eq!(strip_ansi_codes(line_2), " 2  ⋮    │-b = 2");
+        assert_eq!(strip_ansi_codes(line_2), " 2  ⋮    │-b = 23456");
     }
 
     #[test]
@@ -713,7 +713,7 @@ index 223ca50..e69de29 100644
 +++ w/a.py
 @@ -1,2 +0,0 @@
 -a = 1
--b = 2
+-b = 23456
 ";
 
     pub const TWO_PLUS_LINES_DIFF: &str = "\
@@ -724,7 +724,7 @@ index 0000000..223ca50
 +++ i/a.py
 @@ -0,0 +1,2 @@
 +a = 1
-+b = 2
++b = 234567
 ";
 
     pub const ONE_MINUS_ONE_PLUS_LINE_DIFF: &str = "\