Adjust prefix completions to sort alphabetically instead of by length.

Other completions are still sorted by length.
https://github.com/fish-shell/fish-shell/issues/923

path.cpp

@@ -367,6 +367,7 @@ bool path_get_config(wcstring &path)
     return ! result.empty();
 }
 
+__attribute__((unused))
 static void replace_all(wcstring &str, const wchar_t *needle, const wchar_t *replacement)
 {
     size_t needle_len = wcslen(needle);