migrator: move around some migration logic.

Move the checks for moving kegs to a new directory into the relevant
method (`move_to_new_directory`) instead. Also, delete duplicated
directories when we’ve confirmed they are definitely duplicated rather
than telling users to do so.

Library/Homebrew/migrator.rb

@@ -173,27 +173,12 @@ class Migrator
   end
 
   def migrate
-    if old_cellar.exist? && new_cellar.exist?
-      conflicted = false
-      old_cellar.each_child do |c|
-        if (new_cellar/c.basename).exist?
-          conflicted = true
-          onoe "#{new_cellar/c.basename} already exists."
-        end
-      end
-      if conflicted
-        onoe "Remove #{new_cellar} manually and run brew migrate #{oldname}."
-        return
-      end
-    end
-
-    begin
     oh1 "Migrating #{Formatter.identifier(oldname)} to #{Formatter.identifier(newname)}"
     lock
     unlink_oldname
     unlink_newname if new_cellar.exist?
-      move_to_new_directory
     repin
+    move_to_new_directory
     link_oldname_cellar
     link_oldname_opt
     link_newname unless old_linked_keg.nil?
@@ -204,15 +189,34 @@ class Migrator
     onoe "Error occurred while migrating."
     puts e
     puts e.backtrace if ARGV.debug?
-      puts "Backuping..."
+    puts "Backing up..."
     ignore_interrupts { backup_oldname }
   ensure
     unlock
   end
-  end
 
   # move everything from Cellar/oldname to Cellar/newname
   def move_to_new_directory
+    return unless old_cellar.exist?
+
+    if new_cellar.exist?
+      conflicted = false
+      old_cellar.each_child do |c|
+        next unless (new_cellar/c.basename).exist?
+        begin
+          FileUtils.rm_rf c
+        rescue Errno::EACCES
+          conflicted = true
+          onoe "#{new_cellar/c.basename} already exists."
+        end
+      end
+
+      if conflicted
+        onoe "Remove #{new_cellar} manually and run brew migrate #{oldname}."
+        return
+      end
+    end
+
     puts "Moving to: #{new_cellar}"
     if new_cellar.exist?
       FileUtils.mv(old_cellar.children, new_cellar)