Only expand abbreviations if cursor is directly at command token

On a commandline like "ls arg" (cursor at end) we do not expand
abbrevations on enter.  OTOH, on "ls " we do expand. This can be
frustrating because it means that the two obvious ways to suppress
abbrevation expansion (C-Space or post-expansion C-Z) cannot be used to
suppress expansion of a command without arguments.  (One workaround is
"ls #".)

Only expand-on-execute if the cursor is at the command name (no space
in between).

This is a strict improvement for realistic scenarios, because if there
is a space, the user has already expressed the intent to not expand
the abbreviation. (I hope no one is using recursive abbreviations.)

Closes #8423

src/reader.cpp

@@ -3325,7 +3325,7 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
             if (command_test_result == 0 || command_test_result == PARSER_TEST_INCOMPLETE) {
                 // This command is valid, but an abbreviation may make it invalid. If so, we
                 // will have to test again.
-                if (expand_abbreviation_as_necessary(1)) {
+                if (expand_abbreviation_as_necessary(0)) {
                     // Trigger syntax highlighting as we are likely about to execute this command.
                     this->super_highlight_me_plenty();
                     if (conf.syntax_check_ok) {