Make Pathname#atomic_write truly atomic

As we know, files cannot be moved across filesystems atomically. In that
case, FileUtils.mv will make a copy. But if we create the temp file in
the same directory as the target, we can avoid this and use File.rename
directly.

Additionally, the rename should be the absolute last step, so that the
original file is preserved if altering ownership and permissions fails.

Library/Homebrew/extend/pathname.rb

@@ -105,10 +105,9 @@ class Pathname
   # NOTE always overwrites
   def atomic_write content
     require "tempfile"
-    tf = Tempfile.new(basename.to_s)
+    tf = Tempfile.new(basename.to_s, dirname)
     tf.binmode
     tf.write(content)
-    tf.close
 
     begin
       old_stat = stat
@@ -116,16 +115,18 @@ class Pathname
       old_stat = default_stat
     end
 
-    FileUtils.mv tf.path, self
-
     uid = Process.uid
     gid = Process.groups.delete(old_stat.gid) { Process.gid }
 
     begin
-      chown(uid, gid)
-      chmod(old_stat.mode)
+      tf.chown(uid, gid)
+      tf.chmod(old_stat.mode)
     rescue Errno::EPERM
     end
+
+    File.rename(tf.path, self)
+  ensure
+    tf.close!
   end
 
   def default_stat