reinstall: remove partially installed keg.

Previously if a `brew reinstall` was Ctrl-Cd after some files had been
installed it wouldn't try to remove the partially installed keg and the
renaming of the backed-up keg would fail. Alternatively, remove the
partially installed keg as if it has been Ctrl-Cd or otherwise failed
then it's not desirable to keep it.

Library/Homebrew/cmd/reinstall.rb

@@ -66,6 +66,7 @@ module Homebrew
 
     return unless path.directory?
 
+    keg.rmtree if keg.exist?
     path.rename keg
     keg.link unless formula.keg_only?
   end