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

src/libostree/ostree-repo-commit.c

@@ -1442,6 +1442,13 @@ ostree_repo_commit_transaction (OstreeRepo                  *self,
 
   g_return_val_if_fail (self->in_transaction == TRUE, FALSE);
 
+  if ((self->test_error_flags & OSTREE_REPO_TEST_ERROR_PRE_COMMIT) > 0)
+    {
+      g_set_error_literal (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                           "OSTREE_REPO_TEST_ERROR_PRE_COMMIT specified");
+      goto out;
+    }
+
   if (syncfs (self->tmp_dir_fd) < 0)
     {
       glnx_set_error_from_errno (error);