bin: Properly handle a directories named rustfmt.toml

`lookup_project_file` could erroneously find a *directory* named
`rustmfmt.toml` if there was one in its lookup path, and so ignore any
configuration file it should have found further up. The error handling
resulted in this silently using the default configuration.

src/bin/rustfmt.rs

@@ -57,8 +57,11 @@ fn lookup_project_file(input_file: &Path) -> io::Result<PathBuf> {
 
     loop {
         let config_file = current.join("rustfmt.toml");
-        if fs::metadata(&config_file).is_ok() {
-            return Ok(config_file);
+        if let Ok(md) = fs::metadata(&config_file) {
+            // Properly handle unlikely situation of a directory named `rustfmt.toml`.
+            if md.is_file() {
+                return Ok(config_file);
+            }
         }
 
         // If the current directory has no parent, we're done searching.