Add ostree_ensure_repo_writable()

Commands that need to write files within the repo directory can call
this early to ensure the directory is writable for the current user.
If not, it fails with a helpful "You need to be root to perform this
command" message.

src/ostree/ot-main.c

@@ -360,3 +360,17 @@ ostree_admin_option_context_parse (GOptionContext *context,
 out:
   return success;
 }
+
+gboolean
+ostree_ensure_repo_writable (OstreeRepo *repo,
+                             GError **error)
+{
+  gboolean ret;
+
+  ret = ostree_repo_is_writable (repo, error);
+
+  g_prefix_error (error, "Cannot write to repository: ");
+
+  return ret;
+}
+

src/ostree/ot-main.h

@@ -51,3 +51,5 @@ gboolean ostree_admin_option_context_parse (GOptionContext *context,
                                             int *argc, char ***argv,
                                             OstreeSysroot **out_sysroot,
                                             GCancellable *cancellable, GError **error);
+
+gboolean ostree_ensure_repo_writable (OstreeRepo *repo, GError **error);