src/bin/tectonic/v2cli.rs: use as_ref() instead of clone()

This is slightly more efficient.

Also delete the comment, because the intent of the relevant code is
clear enough to not need explanation.

Author: pkgw

src/bin/tectonic/v2cli.rs

@@ -266,8 +266,7 @@ impl BuildCommand {
         setup_options.only_cached(self.only_cached);
 
         for output_name in doc.output_names() {
-            // Added checking to see if the output name matches the specified target name
-            if let Some(out) = self.target.clone() {
+            if let Some(out) = self.target.as_ref() {
                 if out != output_name {
                     continue;
                 }