formatter: allow using "default" terminal color

The "default" color resets the terminal color to default.

CHANGELOG.md

@@ -72,6 +72,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * The progress display on `jj git clone/fetch` now includes the downloaded size.
 
+* The formatter now supports a "default" color that can override another color
+  defined by a parent style.
+
 ### Fixed bugs
 
 * Modify/delete conflicts now include context lines

docs/config.md

@@ -92,8 +92,11 @@ The following colors are available:
 * magenta
 * cyan
 * white
+* default
 
-They each come in a bright version too, e.g. "bright red".
+All of them but "default" come in a bright version too, e.g. "bright red". The
+"default" color can be used to override a color defined by a parent style
+(explained below).
 
 If you use a string value for a color, as in the example above, it will be used
 for the foreground color. You can also set the background color, or make the

src/config-schema.json

@@ -128,6 +128,7 @@
             "definitions": {
                 "colors": {
                     "enum": [
+                        "default",
                         "black",
                         "red",
                         "green",

src/formatter.rs

@@ -372,6 +372,7 @@ fn rules_from_config(config: &config::Config) -> Result<Rules, config::ConfigErr
 
 fn color_for_name(color_name: &str) -> Result<Color, config::ConfigError> {
     match color_name {
+        "default" => Ok(Color::Reset),
         "black" => Ok(Color::Black),
         "red" => Ok(Color::DarkRed),
         "green" => Ok(Color::DarkGreen),
@@ -861,6 +862,36 @@ mod tests {
         @"invalid color: bloo");
     }
 
+    #[test]
+    fn test_color_formatter_normal_color() {
+        // The "default" color resets the color. It is possible to reset only the
+        // background or only the foreground.
+        let config = config_from_string(
+            r#"
+        colors."outer" = {bg="yellow", fg="blue"}
+        colors."outer default_fg" = "default"
+        colors."outer default_bg" = {bg = "default"}
+        "#,
+        );
+        let mut output: Vec<u8> = vec![];
+        let mut formatter = ColorFormatter::for_config(&mut output, &config).unwrap();
+        formatter.push_label("outer").unwrap();
+        formatter.write_str("Blue on yellow, ").unwrap();
+        formatter.push_label("default_fg").unwrap();
+        formatter.write_str(" default fg, ").unwrap();
+        formatter.pop_label().unwrap();
+        formatter.write_str(" and back.\nBlue on yellow, ").unwrap();
+        formatter.push_label("default_bg").unwrap();
+        formatter.write_str(" default bg, ").unwrap();
+        formatter.pop_label().unwrap();
+        formatter.write_str(" and back.").unwrap();
+        insta::assert_snapshot!(String::from_utf8(output).unwrap(),
+        @r###"
+        Blue on yellow,  default fg,  and back.
+        Blue on yellow,  default bg,  and back.
+        "###);
+    }
+
     #[test]
     fn test_color_formatter_sibling() {
         // A partial match on one rule does not eliminate other rules.