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.c

@@ -798,6 +798,9 @@ ostree_repo_init (OstreeRepo *self)
 {
   static gsize gpgme_initialized;
   GLnxLockFile empty_lockfile = GLNX_LOCK_FILE_INIT;
+  const GDebugKey test_error_keys[] = {
+    { "pre-commit", OSTREE_REPO_TEST_ERROR_PRE_COMMIT },
+  };
 
   if (g_once_init_enter (&gpgme_initialized))
     {
@@ -806,6 +809,9 @@ ostree_repo_init (OstreeRepo *self)
       g_once_init_leave (&gpgme_initialized, 1);
     }
 
+  self->test_error_flags = g_parse_debug_string (g_getenv ("OSTREE_REPO_TEST_ERROR"),
+                                                 test_error_keys, G_N_ELEMENTS (test_error_keys));
+
   g_mutex_init (&self->cache_lock);
   g_mutex_init (&self->txn_stats_lock);