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-private.h

@@ -36,6 +36,10 @@ G_BEGIN_DECLS
 #define _OSTREE_SUMMARY_CACHE_DIR "summaries"
 #define _OSTREE_CACHE_DIR "cache"
 
+typedef enum {
+  OSTREE_REPO_TEST_ERROR_PRE_COMMIT = (1 << 0)
+} OstreeRepoTestErrorFlags;
+
 /**
  * OstreeRepo:
  *
@@ -86,6 +90,8 @@ struct OstreeRepo {
   uid_t target_owner_uid;
   gid_t target_owner_gid;
 
+  guint test_error_flags; /* OstreeRepoTestErrorFlags */
+
   GKeyFile *config;
   GHashTable *remotes;
   GMutex remotes_lock;