Make sure redirected stdout and stderr share the same offset

This fixes broken output like this:

  Error: No such file or directory - non_existent_file
  ee/src/tree-1.7.0.tgz
  Already downloaded: /Library/Caches/Homebrew/tree-1.7.0.tgz
  ==> Verifying tree-1.7.0.tgz checksum
  tar xf /Library/Caches/Homebrew/tree-1.7.0.tgz

Library/Contributions/cmd/brew-test-bot.rb

@@ -105,8 +105,10 @@ class Step
     log = log_file_path
 
     pid = fork do
-      STDOUT.reopen(log, "wb")
-      STDERR.reopen(log, "wb")
+      File.open(log, "wb") do |f|
+        STDOUT.reopen(f)
+        STDERR.reopen(f)
+      end
       Dir.chdir(@repository) if @command.first == "git"
       exec(*@command)
     end