fall back to "/tmp" if $TMPDIR isn't set

lib/hub/commands.rb

@@ -366,7 +366,7 @@ module Hub
         url = url.sub(%r{(/pull/\d+)/\w*$}, '\1') unless gist
         ext = gist ? '.txt' : '.patch'
         url += ext unless File.extname(url) == ext
-        patch_file = File.join(ENV['TMPDIR'], "#{gist ? 'gist-' : ''}#{File.basename(url)}")
+        patch_file = File.join(ENV['TMPDIR'] || '/tmp', "#{gist ? 'gist-' : ''}#{File.basename(url)}")
         args.before 'curl', ['-#LA', "hub #{Hub::Version}", url, '-o', patch_file]
         args[idx] = patch_file
       end

test/hub_test.rb

@@ -404,6 +404,13 @@ class HubTest < Test::Unit::TestCase
     end
   end
 
+  def test_am_no_tmpdir
+    with_tmpdir(nil) do
+      cmd = Hub("am https://github.com/defunkt/hub/pull/55").command
+      assert_includes '/tmp/55.patch', cmd
+    end
+  end
+
   def test_am_commit_url
     with_tmpdir('/tmp/') do
       url = 'https://github.com/davidbalbert/hub/commit/fdb9921'