Fetch the default branch when adding a remote from its repo's summary file

Just like we do with the title, fetch the default-branch from the
repository's summary file and use that information when adding a
remote reference from the command line.

https://github.com/flatpak/flatpak/issues/221

app/flatpak-builtins-add-remote.c

@@ -267,6 +267,7 @@ update_remote_with_extra_metadata (FlatpakDir* dir,
                                    GError **error)
 {
   g_autofree char *title = NULL;
+  g_autofree char *default_branch = NULL;
   g_autoptr(GKeyFile) config = NULL;
 
   if (opt_title == NULL)
@@ -279,7 +280,17 @@ update_remote_with_extra_metadata (FlatpakDir* dir,
         opt_title = title;
     }
 
-    if (title != NULL)
+    if (opt_default_branch == NULL)
+    {
+      default_branch = flatpak_dir_fetch_remote_default_branch (dir,
+                                                                remote,
+                                                                NULL,
+                                                                NULL);
+      if (default_branch)
+        opt_default_branch = default_branch;
+    }
+
+    if (title != NULL || default_branch != NULL)
       {
         config = get_config_from_opts (dir, remote);
         return flatpak_dir_modify_remote (dir, remote, config, gpg_data, cancellable, error);

common/flatpak-dir.c

@@ -5260,6 +5260,55 @@ flatpak_dir_fetch_remote_title (FlatpakDir   *self,
   return g_steal_pointer (&title);
 }
 
+char *
+flatpak_dir_fetch_remote_default_branch (FlatpakDir   *self,
+                                         const char   *remote,
+                                         GCancellable *cancellable,
+                                         GError      **error)
+{
+  g_autoptr(GError) my_error = NULL;
+  g_autoptr(GBytes) summary_bytes = NULL;
+  g_autoptr(GVariant) summary = NULL;
+  g_autoptr(GVariant) extensions = NULL;
+  GVariantDict dict;
+  g_autofree char *default_branch = NULL;
+
+  if (error == NULL)
+    error = &my_error;
+
+  if (!flatpak_dir_ensure_repo (self, cancellable, error))
+    return NULL;
+
+  if (!flatpak_dir_remote_fetch_summary (self, remote,
+                                         &summary_bytes,
+                                         cancellable, error))
+    return FALSE;
+
+  if (summary_bytes == NULL)
+    {
+      g_set_error_literal (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                           _("Remote default-branch not available; server has no summary file"));
+      return FALSE;
+    }
+
+  summary = g_variant_ref_sink (g_variant_new_from_bytes (OSTREE_SUMMARY_GVARIANT_FORMAT,
+                                                          summary_bytes, FALSE));
+  extensions = g_variant_get_child_value (summary, 1);
+
+  g_variant_dict_init (&dict, extensions);
+  g_variant_dict_lookup (&dict, "xa.default-branch", "s", &default_branch);
+  g_variant_dict_end (&dict);
+
+  if (default_branch == NULL)
+    {
+      g_set_error_literal (error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND,
+                           _("Remote default-branch not set"));
+      return FALSE;
+    }
+
+  return g_steal_pointer (&default_branch);
+}
+
 static gboolean
 flatpak_dir_parse_summary_for_ref (FlatpakDir   *self,
                                    GVariant     *summary,

common/flatpak-dir.h

@@ -424,6 +424,10 @@ char *   flatpak_dir_fetch_remote_title (FlatpakDir   *self,
                                          const char   *remote,
                                          GCancellable *cancellable,
                                          GError      **error);
+char *   flatpak_dir_fetch_remote_default_branch (FlatpakDir   *self,
+                                                  const char   *remote,
+                                                  GCancellable *cancellable,
+                                                  GError      **error);
 gboolean flatpak_dir_fetch_ref_cache (FlatpakDir   *self,
                                       const char   *remote_name,
                                       const char   *ref,