autocomplete [nfc]: Short-circuit intent-finding on long selection

This is NFC because when this condition is true, the only
return statements below that can actually be reached are
the ones that return null.

In the code as it is, this makes a small optimization.
But it also will help simplify an upcoming refactor.

Author: gnprice

lib/model/autocomplete.dart

@@ -25,6 +25,12 @@ extension ComposeContentAutocomplete on ComposeContentController {
     // in long messages, we bound how far back we look for the intent's start.
     final earliest = max(0, selection.end - 30);
 
+    if (selection.start < earliest) {
+      // The selection extends to before any position we'd consider
+      // for the start of the intent.  So there can't be a match.
+      return null;
+    }
+
     final textUntilCursor = text.substring(0, selection.end);
     for (int position = selection.end - 1; position >= earliest; position--) {
       if (textUntilCursor[position] != '@') {