admin: output redaction fetch-key-list keys to stdout when no output file is specified

Summary: `mononoke_admin redaction fetch-key-list` silently discards the fetched keys when `--output-file` is not provided. This adds an `else` branch to print the keys to stdout.

Differential Revision: D103866028

fbshipit-source-id: f7a76e485b03b1de4cc38702abf84b4c87e73d87

Author: YousefSalama

eden/mononoke/tests/integration/test-redaction-config.t

@@ -84,6 +84,10 @@ Redact file 'c' in commit '$C'
   $ cat "$TESTTMP/keys"
   content.blake2.000a1a9b74aa3da71fcceb653a62cb6987ae440c2b5c3d7e5d08d7c526b1dca8
 
+fetch-key-list without --output-file prints to stdout
+  $ mononoke_admin redaction fetch-key-list -R repo $(cat rs_0)
+  content.blake2.000a1a9b74aa3da71fcceb653a62cb6987ae440c2b5c3d7e5d08d7c526b1dca8
+
 Attempt to redact file 'b' in commit '$COMMIT_B'
 This initially fails because it is still reachable in 'master'
   $ mononoke_admin redaction create-key-list -R repo -i $COMMIT_B b --main-bookmark master_bookmark --skip-aws-sync

eden/mononoke/tools/admin/src/commands/redaction/create_key_list.rs

@@ -138,6 +138,10 @@ pub async fn fetch_key_list(
                     )
                 })?;
         }
+    } else {
+        for key in key_list.keys {
+            println!("{}", key);
+        }
     }
     Ok(())
 }