include leading whitespace in $.message

see helix-editor/helix#1338 (comment)

Leading whitespace is important when injecting diff highlights into
messages trailing the scissors.

Without this change, some adverserial context lines can end up being
mistakenly parsed as non-$.context rules. For example, in the
screenshot of the linked issue comment, a context line is being parsed
by a tricky link that looks like a malformed $.similarity rule. Because
NEWLINE is not a child of $._line but $.source in tree-sitter-git-diff,
part of the line is re-parsed into another valid $._line rule, namely
$.addition in this case.

For an example COMMIT_EDITMSG which has a second diff line 6 characters
to the right of the newline, this commit changes the start column of
the parsed $.message node to include the whitespace:

 (source [0, 0] - [7, 0]
   (subject [0, 0] - [0, 53])
   (comment [2, 0] - [4, 38]
     (scissors [2, 0] - [4, 38]))
   (message [5, 0] - [5, 36])
-  (message [6, 5] - [6, 80]))
+  (message [6, 0] - [6, 80]))

This change probably restricts this grammar to tree-sitter 0.20.1+
because the WHITE_SPACE 'extra' is now used as an extra and within
a rule
(see tree-sitter/tree-sitter#1444 (comment))
but trailing diffs are not meant to be edited anyways, so it's probably
not a big deal.

grammar.js

@@ -51,11 +51,10 @@ module.exports = grammar({
       ),
 
     message: ($) =>
-      choice(
-        seq(
-          choice($.user, /[^\s]+/),
-          optional(repeat(choice($.user, $.item, $._word)))
-        )
+      seq(
+        repeat(WHITE_SPACE),
+        choice($.user, /[^\s]+/),
+        optional(repeat(choice($.user, $.item, $._word)))
       ),
 
     comment: ($) =>

src/grammar.json

@@ -118,51 +118,53 @@
       ]
     },
     "message": {
-      "type": "CHOICE",
+      "type": "SEQ",
       "members": [
         {
-          "type": "SEQ",
+          "type": "REPEAT",
+          "content": {
+            "type": "PATTERN",
+            "value": "[\\t\\f\\v ]+"
+          }
+        },
+        {
+          "type": "CHOICE",
           "members": [
             {
-              "type": "CHOICE",
-              "members": [
-                {
-                  "type": "SYMBOL",
-                  "name": "user"
-                },
-                {
-                  "type": "PATTERN",
-                  "value": "[^\\s]+"
-                }
-              ]
+              "type": "SYMBOL",
+              "name": "user"
             },
             {
-              "type": "CHOICE",
-              "members": [
-                {
-                  "type": "REPEAT",
-                  "content": {
-                    "type": "CHOICE",
-                    "members": [
-                      {
-                        "type": "SYMBOL",
-                        "name": "user"
-                      },
-                      {
-                        "type": "SYMBOL",
-                        "name": "item"
-                      },
-                      {
-                        "type": "SYMBOL",
-                        "name": "_word"
-                      }
-                    ]
+              "type": "PATTERN",
+              "value": "[^\\s]+"
+            }
+          ]
+        },
+        {
+          "type": "CHOICE",
+          "members": [
+            {
+              "type": "REPEAT",
+              "content": {
+                "type": "CHOICE",
+                "members": [
+                  {
+                    "type": "SYMBOL",
+                    "name": "user"
+                  },
+                  {
+                    "type": "SYMBOL",
+                    "name": "item"
+                  },
+                  {
+                    "type": "SYMBOL",
+                    "name": "_word"
                   }
-                },
-                {
-                  "type": "BLANK"
-                }
-              ]
+                ]
+              }
+            },
+            {
+              "type": "BLANK"
             }
           ]
         }