Teach ctest_update to only note the version via CTEST_UPDATE_VERSION_ONLY var.

This allows ctest_update to get the current version without actually changing
the repository. This is useful when using jenkins or an external
project to update the source to a specific version, but you still want
the current version to show up in CDash.
Merge topic 'ctest_update_status_only' into next

e18ed031 Teach ctest_update to only note the version via CTEST_UPDATE_VERSION_ONLY var.