rebase: note `preserve` merges may be a pull config option

rebase: note `preserve` merges may be a pull config option

The `--preserve-merges` option was removed by v2.34.0. However
users may not be aware that it is also a Pull configuration option,
which is still offered by major IDE vendors such as Visual Studio.

Extend the `--preserve-merges` die message to also direct users to
the possible use of the `preserve` option in the `pull.rebase` config.
This is an additional 'belt and braces' information statement.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Junio C Hamano <gitster@pobox.com>