main: fix complete outputting doc page

When calling completion on a nix command containing the word
"--help" nix would first return the entire help page for the
command and then the result of the completion resulting in unusable
output.

By moving the check whether to return when completions were requested
before the check whether help was requested wrappedMain returns
without wrongly printing documentation.

Change-Id: Iedb37434a3ff101f15985319a9a3bcb3f8195796

lix/nix/main.cc

@@ -584,6 +584,8 @@ void mainWrapped(AsyncIoRoot & aio, int argc, char * * argv)
         if (!args.helpRequested && !args.completions) throw;
     }
 
+    if (args.completions) return;
+
     if (args.helpRequested) {
         std::vector<std::string> subcommand;
         MultiCommand * command = &args;
@@ -598,8 +600,6 @@ void mainWrapped(AsyncIoRoot & aio, int argc, char * * argv)
         return;
     }
 
-    if (args.completions) return;
-
     if (args.showVersion) {
         printVersion(programName);
         return;