Merge branch 'explain-0600' into 'master'

Explain the extra chmod

There is confusion about what passing `0600` to File.open does.

```
$ touch /tmp/foobar
$ ls -l /tmp/foobar
-rw-r--r--  1 jacobvosmaer  wheel  0 Sep 26 14:20 /tmp/foobar
$ ruby -e 'File.open("/tmp/foobar", "w", 0600)'
$ ls -l /tmp/foobar
-rw-r--r--  1 jacobvosmaer  wheel  0 Sep 26 14:20 /tmp/foobar
$ 
$ 
$ rm /tmp/foobar
$ ruby -e 'File.open("/tmp/foobar", "w", 0600)'
$ ls -l /tmp/foobar
-rw-------  1 jacobvosmaer  wheel  0 Sep 26 14:21 /tmp/foobar
```

See merge request !6523

lib/gitlab/workhorse.rb

@@ -111,7 +111,7 @@ module Gitlab
       def write_secret
         bytes = SecureRandom.random_bytes(SECRET_LENGTH)
         File.open(secret_path, 'w:BINARY', 0600) do |f|
-          f.chmod(0600)
+          f.chmod(0600) # If the file already existed, the '0600' passed to 'open' above was a no-op.
           f.write(Base64.strict_encode64(bytes))
         end
       end