Simplify logic in RepoCommand

Instead of negative testing before return null, check for when the
command should be enabled and format the string only in this case.

Change-Id: I9fcbf902d62cc76965707a005f0572a8bf75e28f

src/main/java/com/googlesource/gerrit/plugins/download/command/RepoCommand.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/download/command/RepoCommand.java b/src/main/java/com/googlesource/gerrit/plugins/download/command/RepoCommand.java
index 26a40ec..5fa8118 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/download/command/RepoCommand.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/download/command/RepoCommand.java
@@ -33,10 +33,9 @@
 
   @Override
   public String getCommand(DownloadScheme scheme, String project, String ref) {
-    if (!commandAllowed || !(scheme instanceof RepoScheme)) {
-      return null;
+    if (commandAllowed && scheme instanceof RepoScheme) {
+      return scheme.getUrl(project) + " " + ref;
     }
-
-    return scheme.getUrl(project) + " " + ref;
+    return null;
   }
 }