- fix diffFilter when zero lines are styled

With `interactive.diffFilter = delta --color-only` delta is called
by e.g. `git add -p`, but in this mode git hides the terminal from 
the pager.

Plus/minus lines correctly use ANSI sequences to paint up to to the
end of the line, but zero lines always use spaces. This needs the terminal
width, but it is not available for diffFilter. So the fallback
of 80 is used, and zero styles did not extend to the full terminal width.

Since zero lines are only rarely styled (e.g. via
`--zero-style='syntax "#1d1f21" dim'`), this was never noticed.

This also crashed delta when a zero line was longer than 80.

src/paint.rs

@@ -204,7 +204,7 @@ impl<'p> Painter<'p> {
                 self.config,
                 &mut self.line_numbers_data.as_mut(),
                 None,
-                BgShouldFill::With(BgFillMethod::Spaces),
+                BgShouldFill::default(),
             );
         }
     }

src/tests/test_example_diffs.rs

@@ -2137,7 +2137,37 @@ src/align.rs:71: impl<'a> Alignment<'a> { │
         "###);
     }
 
-    const GIT_DIFF_SINGLE_HUNK: &str = "\
+    #[test]
+    fn test_color_only_diff_filter_zero_style_bg() {
+        let result =
+            DeltaTest::with_args(&["--color-only", r##"--zero-style='syntax "#1d1f21" dim'"##])
+                .set_config(|c| c.available_terminal_width = 80)
+                .explain_ansi()
+                .with_input(GIT_DIFF_OF_WIDTH_81);
+
+        assert_snapshot!(result.output, @r###"
+        (normal)
+        --- a.rs
+        +++ b.rs
+        @@ -1,3 +1,3 @@
+        (normal 22)+(231)  (203)if(231) (81)let(231) (149)Ok(231)(foo) { (242)//    Works fine with plus hunk lines which are longer     81(normal)
+        (normal 52)-//                            and it works fine with minus lines, however     81(normal)
+        (dim normal 234) (242)//         styled(!) zero lines can only be as long as the width fallback of 80(normal)
+        (dim normal 234) (231)panic!(); (242)/*     if no tty can be queried, and delta crashes on longer lines: 81(normal)
+        "###);
+    }
+
+    const GIT_DIFF_OF_WIDTH_81: &str = r#"
+--- a.rs
++++ b.rs
+@@ -1,3 +1,3 @@
++  if let Ok(foo) { //    Works fine with plus hunk lines which are longer     81
+-//                            and it works fine with minus lines, however     81
+ //         styled(!) zero lines can only be as long as the width fallback of 80
+ panic!(); /*     if no tty can be queried, and delta crashes on longer lines: 81
+"#;
+
+    const GIT_DIFF_SINGLE_HUNK: &str = "#\
 commit 94907c0f136f46dc46ffae2dc92dca9af7eb7c2e
 Author: Dan Davison <[email protected]>
 Date:   Thu May 14 11:13:17 2020 -0400