rhg: Colored output is not supported

Fallback if it is requested explicitly.

The default is documented as use color "whenever it seems possible".
rhg proceeds without color in that case.

Differential Revision: https://phab.mercurial-scm.org/D11762

rust/rhg/src/main.rs

@@ -28,7 +28,7 b' fn main_with_result('
     repo: Result<&Repo, &NoRepoInCwdError>,
     config: &Config,
 ) -> Result<(), CommandError> {
-    check_unsupported(config)?;
+    check_unsupported(config, ui)?;
 
     let app = App::new("rhg")
         .global_setting(AppSettings::AllowInvalidUtf8)
@@ -617,7 +617,10 b' fn check_extensions(config: &Config) -> '
     }
 }
 
-fn check_unsupported(config: &Config) -> Result<(), CommandError> {
+fn check_unsupported(
+    config: &Config,
+    ui: &ui::Ui,
+) -> Result<(), CommandError> {
     check_extensions(config)?;
 
     if std::env::var_os("HG_PENDING").is_some() {
@@ -634,5 +637,11 b' fn check_unsupported(config: &Config) ->'
         Err(CommandError::unsupported("[decode] config"))?
     }
 
+    if let Some(color) = config.get(b"ui", b"color") {
+        if (color == b"always" || color == b"debug") && !ui.plain() {
+            Err(CommandError::unsupported("colored output"))?
+        }
+    }
+
     Ok(())
 }

tests/test-status-color.t

@@ -1,7 +1,3 b''
-TODO: fix rhg bugs that make this test fail when status is enabled
-  $ unset RHG_STATUS
-
-
   $ cat <<EOF >> $HGRCPATH
   > [ui]
   > color = always