message: Don't try to update the tag after checking it out when building core/sync
  For whatever reason it sometimes breaks, and it can't do anything useful since
  it's a tag and not a branch.
diff: @@ -238,6 +238,7 @@ macro(build_realm_core)
  ExternalProject_Add(realm-core
  PREFIX ${core_prefix_directory}
  BUILD_IN_SOURCE 1
  + UPDATE_DISCONNECTED 1
  INSTALL_COMMAND ""
  CONFIGURE_COMMAND ${CMAKE_COMMAND} -E make_directory build.debug
  && cd build.debug
  @@ -317,6 +318,7 @@ macro(build_realm_sync)
  DEPENDS realm-core
  PREFIX ${cmake_files}/realm-sync
  BUILD_IN_SOURCE 1
  + UPDATE_DISCONNECTED 1
  BUILD_COMMAND ${build_cmd}
  CONFIGURE_COMMAND ""
  INSTALL_COMMAND ""
  