Prevent a panic when uncommenting a line with only a comment token (#5933)

Open a new document `test.rs` and type the following:
`di//<esc><C-c>`

The margin calculation pushes the range out of bounds for the comment
marker when there are no characters (newline) after it.

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value:
Char range out of bounds: char range 0..3,
Rope/RopeSlice char length 2', ropey-1.6.0/src/rope.rs:546:37

The debug build catches the error in the transaction: thread 'main'
panicked at 'attempt to subtract with overflow',
helix-core/src/transaction.rs:503:26

helix-core/src/comment.rs

@@ -45,7 +45,7 @@ fn find_line_comment(
 
             // determine margin of 0 or 1 for uncommenting; if any comment token is not followed by a space,
             // a margin of 0 is used for all lines.
-            if matches!(line_slice.get_char(pos + token_len), Some(c) if c != ' ') {
+            if !matches!(line_slice.get_char(pos + token_len), Some(c) if c == ' ') {
                 margin = 0;
             }
 
@@ -108,8 +108,8 @@ mod test {
         let text = doc.slice(..);
 
         let res = find_line_comment("//", text, 0..3);
-        // (commented = true, to_change = [line 0, line 2], min = col 2, margin = 1)
+        // (commented = true, to_change = [line 0, line 2], min = col 2, margin = 0)
-        assert_eq!(res, (false, vec![0, 2], 2, 1));
+        assert_eq!(res, (false, vec![0, 2], 2, 0));
 
         // comment
         let transaction = toggle_line_comments(&doc, &selection, None);
@@ -136,6 +136,17 @@ mod test {
         assert_eq!(doc, "  1\n\n  2\n  3");
         assert!(selection.len() == 1); // to ignore the selection unused warning
 
+        // 0 margin comments, with no space
+        doc = Rope::from("//");
+        // reset the selection.
+        selection = Selection::single(0, doc.len_chars() - 1);
+
+        let transaction = toggle_line_comments(&doc, &selection, None);
+        transaction.apply(&mut doc);
+        selection = selection.map(transaction.changes());
+        assert_eq!(doc, "");
+        assert!(selection.len() == 1); // to ignore the selection unused warning
+
         // TODO: account for uncommenting with uneven comment indentation
     }
 }