strip non-hack marker when converting back to notebook

Summary:
## What

Fix bug [reported by a user](https://fb.workplace.com/groups/768246428434224/posts/1082807136978150/?comment_id=1083743046884559&reply_comment_id=1084402066818657): When converting notebook->hack we were wrapping python cells in comments, then forgetting to strip those comments when converting hack->notebook.

## implementation:
- We don't want to strip comments from code cells that the user put there, so we use a special `/*non_hack:` comment to distinguish.
- We *could* parse cell metadata to look for the cell language, but we try to limit our dependence on metadata and this way is much simpler

Python cells now look like this when converted to Hack:

```
//bento-cell:{"cell_bento_metadata":{"language":"python"},"cell_type":"code","id":4}
/*non_hack:
%%python

print('Hello from python')
*/
//bento-cell-end
```

Reviewed By: nt591

Differential Revision: D73524434

fbshipit-source-id: cf54fdab111038039bd12711c31ba6682641231a

Author: mheiber

hphp/hack/src/notebook_convert/internal/notebook_chunk.ml

@@ -50,7 +50,26 @@ let cell_end_regexp = Str.regexp ({|^ *|} ^ bento_cell_end_comment)
 
 let is_chunk_end_comment s = Str.string_match cell_end_regexp s 0
 
-let wrap_in_comment = Printf.sprintf "/*\n%s\n*/"
+let non_hack_prefix = "/*@non_hack:\n"
+
+let non_hack_suffix = "\n*/"
+
+let strip_non_hack_comment (code : string) =
+  if
+    String.is_prefix code ~prefix:non_hack_prefix
+    && String.is_suffix code ~suffix:non_hack_suffix
+  then
+    String.sub
+      code
+      ~pos:(String.length non_hack_prefix)
+      ~len:
+        (String.length code
+        - String.length non_hack_prefix
+        - String.length non_hack_suffix)
+  else
+    code
+
+let wrap_in_comment : string -> string = Printf.sprintf "/*@non_hack:\n%s\n*/"
 
 let to_hack { chunk_kind; id; contents; cell_bento_metadata } : string =
   let (cell_type, body) =
@@ -126,7 +145,11 @@ let metadata_of_comment (comment : string) :
     Error (Notebook_convert_error.Invalid_input msg)
   | metadata -> Ok metadata
 
-let strip_comment_wrapper (hack : string) : string =
+(** remove leading `/*\n` and trailing `\n*/`. These were inserted
+* by older versions  of the conversion script for non-hack cells.
+* We also handle the newer format of `/*@non_hack:\n` ... `\n*/`
+*)
+let strip_legacy_comment_wrapper (hack : string) : string =
   let stripped = String.strip hack in
   String.sub
     stripped
@@ -138,12 +161,13 @@ let of_hack ~(comment : string) (hack : string) :
   Result.map
     (metadata_of_comment comment)
     ~f:(fun { metadata_id = id; metadata_cell_type; cell_bento_metadata } ->
+      let hack = strip_non_hack_comment hack in
       if String.equal "code" metadata_cell_type then
         { id; chunk_kind = Hack; contents = hack; cell_bento_metadata }
       else
         {
           id;
           chunk_kind = Non_hack { cell_type = metadata_cell_type };
-          contents = strip_comment_wrapper hack;
+          contents = strip_legacy_comment_wrapper hack;
           cell_bento_metadata;
         })

hphp/hack/test/notebook_convert/hack_to_notebook/nb1.php

@@ -10,6 +10,14 @@
 */
 //@bento-cell-end
 
+//@bento-cell:{"cell_bento_metadata":{"language":"python"},"cell_type":"code","id":4}
+/*@non_hack:
+%%python
+
+print('Hello from python')
+*/
+//@bento-cell-end
+
 //@bento-cell:{"id": 1, "cell_type": "code"}
 class N1234MyClass {
   public function hello(): void {

hphp/hack/test/notebook_convert/hack_to_notebook/nb1.php.exp

@@ -30,6 +30,13 @@
       "metadata":{},
       "outputs":[],
       "source":["echo \"hi2\";\n","$m->hello();"]
+    },
+    {
+      "cell_type":"code",
+      "execution_count":null,
+      "metadata":{"language":"python"},
+      "outputs":[],
+      "source":["%%python\n","print('Hello from python')"]
     }
   ],
   "metadata":{

hphp/hack/test/notebook_convert/hack_to_notebook/nb11_bad_notebook_number.php

@@ -3,7 +3,7 @@
 //@bento-notebook:{"notebook_number":"1234","kernelspec":{"display_name":"hack","language":"hack","name":"bento_kernel_hack"}}
 
 //@bento-cell:{"id": 2, "cell_type": "markdown"}
-/*
+/*@non_hack:
 # Check it out
 
 I am a *markdown* **cell**

hphp/hack/test/notebook_convert/hack_to_notebook/nb2_invalid_metadata.php

@@ -2,7 +2,7 @@
 //@bento-notebook:{"notebook_number":"N1234","kernelspec":{"display_name":"hack","language":"hack","name":"bento_kernel_hack"}}
 
 //@bento-cell:{"id": 2, "cell_type": "markdown"}
-/*
+/*@non_hack:
 # Check it out
 
 I am a *markdown* **cell**

hphp/hack/test/notebook_convert/hack_to_notebook/nb3_invalid_metadata.php

@@ -2,7 +2,7 @@
 //@bento-notebook:{"notebook_number":"N1234","kernelspec":{"display_name":"hack","language":"hack","name":"bento_kernel_hack"}}
 
 //@bento-cell:{"id": 2, "cell_type": "markdown"}
-/*
+/*@non_hack:
 # Check it out
 
 I am a *markdown* **cell**

hphp/hack/test/notebook_convert/hack_to_notebook/nb4_invalid_metadata.php

@@ -3,7 +3,7 @@
 //@bento-notebook:{"notebook_number":" i am not valid json
 
 //@bento-cell:{"id": 2, "cell_type": "markdown"}
-/*
+/*@non_hack:
 # Check it out
 
 I am a *markdown* **cell**

hphp/hack/test/notebook_convert/hack_to_notebook/nb7_missing_end_of_cell_marker_best_effort.php

@@ -3,7 +3,7 @@
 //@bento-notebook:{"notebook_number":"N1234","kernelspec":{"display_name":"hack","language":"hack","name":"bento_kernel_hack"}}
 
 //@bento-cell:{"id": 2, "cell_type": "markdown"}
-/*
+/*@non_hack:
 # Check it out
 
 I am a *markdown* **cell**

hphp/hack/test/notebook_convert/hack_to_notebook/nb7_missing_end_of_cell_marker_best_effort.php.exp

@@ -21,7 +21,7 @@
       "execution_count":null,
       "metadata":{},
       "outputs":[],
-      "source":["# Check it out\n","I am a *markdown* **cell**"]
+      "source":["non_hack:\n","# Check it out\n","I am a *markdown* **cell**"]
     },
     {
       "cell_type":"code",

hphp/hack/test/notebook_convert/notebook_to_hack/nb1.tc.ipynb.php

@@ -7,7 +7,7 @@ class N1234MyClass {}
 //@bento-cell-end
 
 //@bento-cell:{"cell_bento_metadata":{},"cell_type":"markdown","id":3}
-/*
+/*@non_hack:
 ## Markdown cell (cell 3)
 
 I am a *markdown* **cell**