t6300: fix setup with GPGSSH but without GPG

In a test introduced by 26c9c03 (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

t/t6300-for-each-ref.sh

@@ -1584,7 +1584,8 @@ test_expect_success GPGSSH 'setup for signature atom using ssh' '
 	test_config user.signingkey "${GPGSSH_KEY_PRIMARY}" &&
 	echo "8" >file &&
 	test_tick &&
-	git commit -a -S -m "file: 8" &&
+	git add file &&
+	git commit -S -m "file: 8" &&
 	git tag eighth-signed-ssh
 '