Change the lookahead in MacroParser::new.

As it happens, lookahead values of 0, 1, and 2 all work fine here, due
to the structure of the code. (Values or 3 or greater cause test
failures.) This commit changes the lookahead to zero because that will
facilitate cleanups in subsequent commits.

Author: nnethercote

src/tools/rustfmt/src/macros.rs

@@ -1190,7 +1190,7 @@ impl<'a> MacroParser<'a> {
     // (`(` ... `)` `=>` `{` ... `}`)*
     fn parse(&mut self) -> Option<Macro> {
         let mut branches = vec![];
-        while self.toks.look_ahead(1).is_some() {
+        while self.toks.look_ahead(0).is_some() {
             branches.push(self.parse_branch()?);
         }