emacs: Invoke gpg with --batch and --no-tty

emacs: Invoke gpg with --batch and --no-tty

When invoking gpg as a backgrounded tool, it's important to let gpg
know that it is backgrounded, to avoid spurious prompts or other
breakage.

In particular, https://bugs.debian.org/913614 was a regression in
GnuPG which causes problems when importing keys without a terminal,
but gpg expects one.

Ensuring that notmuch-emacs always invokes gpg as a background process
should avoid some of these unnecessary failure.

Thanks to Justus Winter for finding this problem.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>