Prefer execution when there's only a single line entered

Closes gh-10425

The heuristic here is to treat a single line specially, and always
evaluate it as if the cursor was at the end. An alternative heuristic
could be to do this if the cursor is on the last line of the input.

IPython/terminal/shortcuts.py

@@ -108,8 +108,13 b' def newline_or_execute_outer(shell):'
                 b.cancel_completion()
             return
 
-        before_text = d.text[:d.cursor_position]
-        status, indent = shell.input_splitter.check_complete(before_text + '\n')
+        # If there's only one line, treat it as if the cursor is at the end.
+        # See https://github.com/ipython/ipython/issues/10425
+        if d.line_count == 1:
+            check_text = d.text
+        else:
+            check_text = d.text[:d.cursor_position]
+        status, indent = shell.input_splitter.check_complete(check_text + '\n')
 
         if not (d.on_last_line or
                 d.cursor_position_row >= d.line_count - d.empty_line_count_at_the_end()