t6300: fix setup with GPGSSH but without GPG

t6300: fix setup with GPGSSH but without GPG

In a test introduced by 26c9c03f0a (ref-filter: add new "signature"
atom, 2023-06-04) the file named "file" is added by a setup step that
requires GPG and modified by a second setup step that requires GPGSSH.
Systems lacking the first prerequisite skip the initial setup step and
then "git commit -a" in the second one doesn't find the modified file.
Add it explicitly.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>