repo: Never delete .commitmeta files

Do not delete a .commitmeta file after removing the last metadata entry.
This way a client will pull the empty .commitmeta file and overwrite old
metadata as expected.

https://bugzilla.gnome.org/750459

tests/test-commit-sign.sh

@@ -119,4 +119,13 @@ ${CMD_PREFIX} ostree --repo=${test_tmpdir}/ostree-srv/gnomerepo gpg-sign --gpg-h
 ${CMD_PREFIX} ostree --repo=repo pull origin main
 ${CMD_PREFIX} ostree --repo=repo show main | grep -o 'Found [[:digit:]] signature' > show
 assert_file_has_content show 'Found 1 signature'
+
+# Delete the signature from the commit so the detached metadata is empty,
+# then pull and verify the signature is also deleted on the client side.
+${CMD_PREFIX} ostree --repo=${test_tmpdir}/ostree-srv/gnomerepo gpg-sign --gpg-homedir=${SRCDIR}/gpghome --delete main $keyid
+${CMD_PREFIX} ostree --repo=repo pull origin main
+if ${CMD_PREFIX} ostree --repo=repo show main | grep -o 'Found [[:digit:]] signature'; then
+  assert_not_reached
+fi
+
 rm -rf repo gnomerepo-files