cli: ability to disable default colors

My motivation is mostly to address #5098, even as a stopgap meassure.

CHANGELOG.md

@@ -143,6 +143,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * Conditional configuration now supports `--when.commands` to change configuration
   based on subcommand.
 
+* New `JJ_NO_DEFAULT_COLORS` environment variable disables setting build-in default colors.
+
 ### Fixed bugs
 
 * `jj git fetch` with multiple remotes will now fetch from all remotes before

cli/src/config.rs

@@ -488,13 +488,23 @@ pub fn default_config_layers() -> Vec<ConfigLayer> {
     // Syntax error in default config isn't a user error. That's why defaults are
     // loaded by separate builder.
     let parse = |text: &'static str| ConfigLayer::parse(ConfigSource::Default, text).unwrap();
-    let mut layers = vec![
-        parse(include_str!("config/colors.toml")),
-        parse(include_str!("config/merge_tools.toml")),
-        parse(include_str!("config/misc.toml")),
-        parse(include_str!("config/revsets.toml")),
-        parse(include_str!("config/templates.toml")),
-    ];
+    let mut layers = {
+        let maybe_colors = if env::var("JJ_NO_DEFAULT_COLORS").is_ok() {
+            vec![]
+        } else {
+            vec![parse(include_str!("config/colors.toml"))]
+        };
+        [
+            maybe_colors,
+            vec![
+                parse(include_str!("config/merge_tools.toml")),
+                parse(include_str!("config/misc.toml")),
+                parse(include_str!("config/revsets.toml")),
+                parse(include_str!("config/templates.toml")),
+            ],
+        ]
+        .concat()
+    };
     if cfg!(unix) {
         layers.push(parse(include_str!("config/unix.toml")));
     }