rust-chg: suppress panic while writing chg error to stderr

rust-chg: suppress panic while writing chg error to stderr

Otherwise "chg >/dev/full 2>&1" would exit with 101. Spotted by test-basic.t.

--- a/rust/chg/src/main.rs	Sun Oct 14 04:37:25 2018 -0400
+++ b/rust/chg/src/main.rs	Sun Oct 07 20:55:51 2018 +0900
@@ -20,7 +20,7 @@
 
 fn main() {
     let code = run().unwrap_or_else(|err| {
-        eprintln!("chg: abort: {}", err);
+        writeln!(io::stderr(), "chg: abort: {}", err).unwrap_or(());
         255
     });
     process::exit(code);