cmd/gist-logs: Fix bug with parsing of command-line arguments

- A user reported the following error when running `brew gist-logs`,
reproducible with any formula:

```
$ brew gist-logs rustup-init
Error: undefined method `length' for nil:NilClass
Please report this bug:
  https://docs.brew.sh/Troubleshooting
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/cmd/gist-logs.rb:145:in `gist_logs'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:102:in
```

- From what I can tell, `gist_logs_args.parse` had ended up in the wrong
  method. The `gistify_logs` method already has `formula` (ie the
  formula name) passed into it. But it's the `gist_logs` method that needs
  to do the argument parsing. This was discovered by looking at some other
  recently changed ARGV => Homebrew.args code like cmd/info.

Library/Homebrew/cmd/gist-logs.rb

@@ -33,8 +33,6 @@ module Homebrew
   end
 
   def gistify_logs(f)
-    gist_logs_args.parse
-
     files = load_logs(f.logs)
     build_time = f.logs.ctime
     timestamp = build_time.strftime("%Y-%m-%d_%H-%M-%S")
@@ -142,6 +140,8 @@ module Homebrew
   end
 
   def gist_logs
+    gist_logs_args.parse
+
     raise FormulaUnspecifiedError if Homebrew.args.resolved_formulae.length != 1
 
     Install.perform_preinstall_checks(all_fatal: true)