Digging in to some of the meatier backlog today. Backlog down to 225.

A lot of fixes around using `git annex enableremote` to add new gpg keys to
a gcrypt special remote. 

Had to make git-annex's use of `GIT_SSH`/`GIT_SSH_COMMAND`
contingent on `GIT_ANNEX_USE_GIT_SSH=1` being set. Unfortunate, but
difference from git made at least one existing use of that environment
variable break, and so it will need to be whitelisted in places where
git-annex should use it.

Added support for `git annex add --update`

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Today's work was sponsored by Trenton Cronholm on Patreon.
