fsck: Check for refs missing corresponding commit

Just doing this one quickly since it was easy.

Closes: https://github.com/ostreedev/ostree/issues/831

Closes: #841
Approved by: jlebon

tests/test-corruption.sh

@@ -19,10 +19,12 @@
 
 set -euo pipefail
 
-echo "1..2"
+echo "1..3"
 
 . $(dirname $0)/libtest.sh
 
+cd ${test_tmpdir}
+rm repo files -rf
 setup_test_repository "bare"
 $OSTREE checkout test2 checkout-test2
 cd checkout-test2
@@ -34,6 +36,8 @@ $OSTREE fsck -q
 echo "ok chmod"
 
 cd ${test_tmpdir}
+rm repo files -rf
+setup_test_repository "bare"
 rm checkout-test2 -rf
 $OSTREE checkout test2 checkout-test2
 cd checkout-test2
@@ -41,3 +45,14 @@ chmod o+x firstfile
 $OSTREE fsck -q --delete && (echo 1>&2 "fsck unexpectedly succeeded"; exit 1)
 
 echo "ok chmod"
+
+cd ${test_tmpdir}
+rm repo files -rf
+setup_test_repository "bare"
+find repo/ -name '*.commit' -delete
+if $OSTREE fsck -q 2>err.txt; then
+    assert_not_reached "fsck unexpectedly succeeded"
+fi
+assert_file_has_content_literal err.txt "Loading commit for ref test2: No such metadata object"
+
+echo "ok missing commit"