remotenames: enable the storage config option if extension is enabled
Before this patch, the config option to store remotenames was set to False by
default and remotenames extension does not set it True. So if you enable
remotenames extension without setting 'experimental.remotenames=True', you won't
get the remotenames.
This patch makes remotenames enable the config option if then extension is
enabled which makes sense.
Differential Revision: https://phab.mercurial-scm.org/D3520