Refine "arc land" behavior when pushing "onto" a new branch

Summary:
Ref T13546. If the "onto" branch doesn't exist yet and has a "/" in it, we need to preface it with "refs/heads" explicitly.

Fix a "false/null" issue with argument validation.

Possibly, "arc land" should prompt you before creating branches in the remote.

Test Plan: Ran "arc land --onto does-not-exist/1.1" and created a branch.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21341

src/land/engine/ArcanistGitLandEngine.php

@@ -885,6 +885,11 @@ final class ArcanistGitLandEngine
             $onto_ref));
       }
     }
+
+    // TODO: Check that these refs really exist in the remote? Checking the
+    // remote is expensive and users probably rarely specify "--onto" manually,
+    // but if "arc land" creates branches without prompting when you make typos
+    // that also seems questionable.
   }
 
   protected function selectOntoRefs(array $symbols) {
@@ -1491,7 +1496,7 @@ final class ArcanistGitLandEngine
 
     foreach ($this->getOntoRefs() as $onto_ref) {
       $refspecs[] = sprintf(
-        '%s:%s',
+        '%s:refs/heads/%s',
         $this->getDisplayHash($into_commit),
         $onto_ref);
     }

src/land/engine/ArcanistLandEngine.php

@@ -1361,7 +1361,7 @@ abstract class ArcanistLandEngine
     }
 
     $into = $this->getIntoArgument();
-    if ($into && ($into_empty !== null)) {
+    if ($into && $into_empty) {
       throw new PhutilArgumentUsageException(
         pht(
           'Arguments "--into" and "--into-empty" are mutually exclusive.'));