Option to set the background extension mode to ANSI or spaces

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.

The command line option --line-fill-method ansi|spaces can change
how the right panel background is filled.

Add enums `BgShouldFill` and `BgFillMethod` to better distinguish
if the background should be filled, and if so, how.

src/cli.rs

@@ -526,6 +526,11 @@ pub struct Opt {
     /// delta will be slow on very long lines (e.g. minified .js) if truncation is disabled.
     pub max_line_length: usize,
 
+    /// How to extend the background color to the end of the line in side-by-side mode. Can
+    /// be ansi (default) or spaces. Has no effect if --width=variable is given.
+    #[structopt(long = "line-fill-method")]
+    pub line_fill_method: Option<String>,
+
     /// The width of underline/overline decorations. Use --width=variable to extend decorations and
     /// background colors to the end of the text only. Otherwise background colors extend to the
     /// full terminal width.

src/config.rs

@@ -16,6 +16,7 @@ use crate::env;
 use crate::features::navigate;
 use crate::features::side_by_side;
 use crate::git_config::{GitConfig, GitConfigEntry};
+use crate::paint::BgFillMethod;
 use crate::style::{self, Style};
 
 pub struct Config {
@@ -51,6 +52,7 @@ pub struct Config {
     pub hyperlinks_file_link_format: String,
     pub inspect_raw_lines: cli::InspectRawLines,
     pub keep_plus_minus_markers: bool,
+    pub line_fill_method: BgFillMethod,
     pub line_numbers: bool,
     pub line_numbers_left_format: String,
     pub line_numbers_left_style: Style,
@@ -187,6 +189,16 @@ impl From<cli::Opt> for Config {
         let file_renamed_label = opt.file_renamed_label;
         let hunk_label = opt.hunk_label;
 
+        let line_fill_method = match opt.line_fill_method.as_deref() {
+            // Note that "default" is not documented
+            Some("ansi") | Some("default") | None => BgFillMethod::TryAnsiSequence,
+            Some("spaces") => BgFillMethod::Spaces,
+            _ => {
+                eprintln!("Invalid option for line-fill-method: Expected \"ansi\" or \"spaces\".");
+                process::exit(1);
+            }
+        };
+
         let navigate_regexp = if opt.navigate || opt.show_themes {
             Some(navigate::make_navigate_regexp(
                 opt.show_themes,
@@ -245,6 +257,7 @@ impl From<cli::Opt> for Config {
             hyperlinks_file_link_format: opt.hyperlinks_file_link_format,
             inspect_raw_lines: opt.computed.inspect_raw_lines,
             keep_plus_minus_markers: opt.keep_plus_minus_markers,
+            line_fill_method,
             line_numbers: opt.line_numbers,
             line_numbers_left_format: opt.line_numbers_left_format,
             line_numbers_left_style,

src/features/line_numbers.rs

@@ -383,7 +383,7 @@ 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]
@@ -407,7 +407,7 @@ 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]
@@ -489,7 +489,7 @@ 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]
@@ -514,7 +514,7 @@ index 223ca50..e69de29 100644
 +++ w/a.py
 @@ -1,2 +0,0 @@
 -a = 1
--b = 2
+-b = 23456
 ";
 
     pub const TWO_PLUS_LINES_DIFF: &str = "\
@@ -525,7 +525,7 @@ 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 = "\