migrator: allow new cellar to exist already

Lets us migrate a formula to a name that may have previously been used.

If gnupg 1.x is installed as "gnupg" and gnupg 2.x is installed as
"gnupg2," it's currently not possible to rename gnupg2 -> gnupg, since
the 1.4 keg will already be installed in the "gnupg" Cellar, so in order
to reclaim the name "gnupg" to be used for 2.1, either 1.x must be
manually uninstalled, or the new cellar needs to be allowed to exist
already.

Library/Homebrew/migrator.rb

@@ -147,15 +147,25 @@ class Migrator
   end
 
   def migrate
-    if new_cellar.exist?
+    if old_cellar.exist? && new_cellar.exist?
-      onoe "#{new_cellar} already exists; remove it manually and run brew migrate #{oldname}."
+      conflicted = false
-      return
+      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
       link_oldname_cellar
@@ -178,7 +188,11 @@ class Migrator
   # move everything from Cellar/oldname to Cellar/newname
   def move_to_new_directory
     puts "Moving to: #{new_cellar}"
-    FileUtils.mv(old_cellar, new_cellar)
+    if new_cellar.exist?
+      FileUtils.mv(old_cellar.children, new_cellar)
+    else
+      FileUtils.mv(old_cellar, new_cellar)
+    end
   end
 
   def repin
@@ -207,6 +221,14 @@ class Migrator
     end
   end
 
+  def unlink_newname
+    oh1 "Unlinking #{Formatter.identifier(newname)}"
+    new_cellar.subdirs.each do |d|
+      keg = Keg.new(d)
+      keg.unlink
+    end
+  end
+
   def link_newname
     oh1 "Linking #{Formatter.identifier(newname)}"
     new_keg = Keg.new(new_linked_keg_record)