repo: Add OSTREE_REPO_TEST_ERROR=pre-commit env var

Setting this causes commit to error out.  There are other ways we
could do this in a more sophisticated fashion, such as via SystemTap
etc.  But this has low-tech applicablity, works as non-root.

The reason I'm adding this is so that we can add test cases for
cleanup of the `tmp/staging-` directory.

Closes: #170
Approved by: jlebon

tests/basic-test.sh

@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..53"
+echo "1..54"
 
 $OSTREE checkout test2 checkout-test2
 echo "ok checkout"
@@ -397,6 +397,15 @@ assert_file_has_content test2-checkout/baz/cow moo
 assert_has_dir repo2/uncompressed-objects-cache
 echo "ok disable cache checkout"
 
+cd ${test_tmpdir}
+rm checkout-test2 -rf
+$OSTREE checkout test2 checkout-test2
+if env OSTREE_REPO_TEST_ERROR=pre-commit $OSTREE commit -b test2 -s '' $test_tmpdir/checkout-test2 2>err.txt; then
+    assert_not_reached "Should have hit OSTREE_REPO_TEST_ERROR_PRE_COMMIT"
+fi
+assert_file_has_content err.txt OSTREE_REPO_TEST_ERROR_PRE_COMMIT
+echo "ok test error pre commit"
+
 # Whiteouts
 cd ${test_tmpdir}
 mkdir -p overlay/baz/