This is groundwork for making special remotes like borg be skipped by
sync when on an offline drive.
Added AVAILABILITY UNAVAILABLE reponse and the UNAVAILABLERESPONSE extension
to the external special remote protocol. The extension is needed because
old git-annex, if it sees that response, will display a warning
message. (It does continue as if the remote is globally available, which
is acceptable, and the warning is only displayed at initremote due to
remote.name.annex-availability caching, but still it seemed best to make
this a protocol extension.)
The remote.name.annex-availability git config is no longer used any
more, and is documented as such. It was only used by external special
remotes to cache the availability, to avoid needing to start the
external process every time. Now that availability is queried as an
Annex action, the external is only started by sync (and the assistant),
when they actually check availability.
Sponsored-by: Nicholas Golder-Manning on Patreon
git-annex allows managing large files with git, without storing the file
contents in git. It can sync, backup, and archive your data, offline
and online. Checksums and encryption keep your data safe and secure. Bring
the power and distributed nature of git to bear on your large files with
git-annex.
For documentation, see doc/ or <https://git-annex.branchable.com/>
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