bundlespec: check the `obsolescence` value before adding the caps This does not really matters as the logic to decide wether or not adding the part is correct and elsewhere. However this seems like a good idea to align this logic witht he semantic of the option.

bundlespec: handle the presence of obsmarker part This make `hg debugbundle --spec` more informative about extra part in the bundle.

bundlespec: fix the generation of bundlespec for `cg.version` If the value is non-default, we display it.