rhg: signal when falling back in logs

We use the `trace` level for the actual message because it can get really busy.

Differential Revision: https://phab.mercurial-scm.org/D11814

rust/rhg/src/main.rs

@@ -1,6 +1,6 b''
 extern crate log;
 use crate::error::CommandError;
-use crate::ui::Ui;
+use crate::ui::{local_to_utf8, Ui};
 use clap::App;
 use clap::AppSettings;
 use clap::Arg;
@@ -383,7 +383,7 b' fn exit('
 ) -> ! {
     if let (
         OnUnsupported::Fallback { executable },
-        Err(CommandError::UnsupportedFeature { .. }),
+        Err(CommandError::UnsupportedFeature { message }),
     ) = (&on_unsupported, &result)
     {
         let mut args = std::env::args_os();
@@ -413,6 +413,8 b' fn exit('
             ));
             on_unsupported = OnUnsupported::Abort
         } else {
+            log::debug!("falling back (see trace-level log)");
+            log::trace!("{}", local_to_utf8(message));
             // `args` is now `argv[1..]` since we’ve already consumed
             // `argv[0]`
             let mut command = Command::new(executable_path);

rust/rhg/src/ui.rs

@@ -219,6 +219,12 b' pub fn utf8_to_local(s: &str) -> Cow<[u8'
     Cow::Borrowed(bytes)
 }
 
+/// Decode user system bytes to Rust string.
+pub fn local_to_utf8(s: &[u8]) -> Cow<str> {
+    // TODO decode from the user's system
+    String::from_utf8_lossy(s)
+}
+
 /// Should formatted output be used?
 ///
 /// Note: rhg does not have the formatter mechanism yet,