Sometimes users would get confused because an option they were looking
for was not mentioned on a subcommand's man page, and they had not
noticed that the main git-annex man page had a list of common options.
This change lets each subcommand mention the common options, similarly
to how the matching options are handled.
This commit was sponsored by Svenne Krap on Patreon.
As long as the class of remotes supports exporting, it's tested whether
or not the remote is configured with exporttree=yes.
Also, made testremote of a remote configured with exporttree=yes
disable that configuration for testing non-export storage.
This commit was supported by the NSF-funded DataLad project.
