Bump the version of rustfmt used in tidy

To pick up rust-lang/rustfmt#4461
So that rustfmt has the parsing fix from #76274

...and do a reformat that it wants.

Author: scottmcm

library/core/src/iter/adapters/mod.rs

@@ -2381,11 +2381,7 @@ where
         }
 
         let n = self.len();
-        if n == 0 {
-            try { init }
-        } else {
-            self.iter.try_rfold(init, check(n, fold)).into_try()
-        }
+        if n == 0 { try { init } } else { self.iter.try_rfold(init, check(n, fold)).into_try() }
     }
 
     fn rfold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc

src/stage0.txt

@@ -20,7 +20,7 @@ cargo: beta
 # bootstrapping issues with use of new syntax in this repo. If you're looking at
 # the beta/stable branch, this key should be omitted, as we don't want to depend
 # on rustfmt from nightly there.
-rustfmt: nightly-2020-10-07
+rustfmt: nightly-2020-10-12
 
 # When making a stable release the process currently looks like:
 #