This option avoids gpg key distribution, at the expense of flexability, and
with the requirement that all clones of the git repository be equally
trusted.
Fails with gpg 2. Instead, use a different environment variable.
The clean fix would instead be to add an annex.gpg-options configuration.
But, that would be rather a lot of work and it's unlikely it would be
useful for much else.
Only set --no-tty when GPG_AGENT_INFO is set and batch mode is used.
In the test suite, set GPG_AGENT_INFO to /dev/null to avoid the test suite
relying on /dev/tty.
