reader: Return true if suppress-autosuggestion suppressed

This allows

bind -k backspace suppress-autosuggestion or backward-delete-char

To remove the suggestion on the first press and then delete
chars.

Note: This requires that we then don't reenable suggestions
immediately afterwards. Currently we don't after deletion.

Fixes #1419.

doc_src/cmds/bind.rst

@@ -174,7 +174,7 @@ The following special input functions are available:
 
 - ``self-insert-notfirst``, inserts the matching sequence into the command line, unless the cursor is at the beginning
 
-- ``suppress-autosuggestion``, remove the current autosuggestion
+- ``suppress-autosuggestion``, remove the current autosuggestion. Returns true if there was a suggestion to remove.
 
 - ``swap-selection-start-stop``, go to the other end of the highlighted text without changing the selection
 

src/reader.cpp

@@ -3298,7 +3298,10 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
         }
         case rl::suppress_autosuggestion: {
             suppress_autosuggestion = true;
+            bool success = !autosuggestion.empty();
             autosuggestion.clear();
+            // Return true if we had a suggestion to clear.
+            inputter.function_set_status(success);
             break;
         }
         case rl::accept_autosuggestion: {