Stop rewriting the history file on every command

fish is designed to append to the history file in most cases. However
save_internal_via_appending was never returning success, so we were
always doing the slow rewrite path. Correctly return success.

Fixes #6042

CHANGELOG.md

@@ -23,6 +23,7 @@
 - `switch` now allows arguments that expand to nothing, like empty variables (#5677).
 - The null command (:) now always exits successfully, rather than echoing last return code.
 - Cursor configuration instructions for vi-mode have been added to the fish documentation.
+- A bug caused fish to rewrite the history file on every command. This has been fixed: fish will append to the file in most cases (#6042).
 
 ### Syntax changes and new commands
 - Brace expansion now only takes place if the braces include a "," or a variable expansion, so things like `git reset HEAD@{0}` now work (#5869).

src/history.cpp

@@ -947,6 +947,8 @@ bool history_impl_t::save_internal_via_appending() {
         this->history_file_id = file_id_for_fd(history_fd);
 
         close(history_fd);
+
+        ok = (err == 0);
     }
 
     // If someone has replaced the file, forget our file state.