[toplevel] fix double-printing of infer options in --help

Summary:
The list of argument specs is a global ref inside `CommandLineOptions`, which
need to be reset to the empty list every time `parse` is called. Otherwise, we
get duplicated sections:

```
$ infer --help
Infer version v0.9.4-84d61cb
Copyright 2009 - present Facebook. All Rights Reserved.

  Toplevel options

  --inferconfig-home <dir>   Path to the .inferconfig file
  --project-root | -pr <dir> Specify the root directory of the project (default: /home/jul/infer)

  Analysis (backend) options

  Clang frontend options

  Java frontend options

  Toplevel options

[... the rest of the options -- without --inferconfig-home or --project-root,
      with all the section headers again ...]
```

Reviewed By: jberdine

Differential Revision: D4333448

fbshipit-source-id: f91ea66

infer/src/base/CommandLineOption.ml

@@ -621,6 +621,8 @@ let parse ?(incomplete=false) ?(accept_unknown=false) ?config_file current_exe e
   (* speclist includes args for current exe with docs, and all other args without docs, so
      that all args can be parsed, but --help and parse failures only show external args for
      current exe *)
+  (* reset the speclist between calls to this function *)
+  curr_speclist := [];
   if current_exe = Toplevel then
     add_to_curr_speclist ~add_help:true ~header:"Toplevel options" current_exe
   else