Auto merge of #1121 - mikhail-m1:is_tracking, r=Diggsey

fix ToolchainDesc is_tracking

issue #756.

I wonder, why is not ToolchainDesc.channel a enum?

Author: bors

src/rustup-dist/src/dist.rs

@@ -375,7 +375,8 @@ impl ToolchainDesc {
     }
 
     pub fn is_tracking(&self) -> bool {
-        self.date.is_none()
+        let channels = ["nightly", "beta", "stable"];
+        channels.iter().any(|x| *x == self.channel) && self.date.is_none()
     }
 }