[MERGE] proper gpg-agent support

[John Arbash Meinel]

A while ago I wrote a plugin to spawn gpg in a different way, so that it
could connect to pinentry-curses properly. (Basically, gpg has an issue
if stdin is not a tty, which is weird, because it works just fine for
standard password prompting).

Anyway, James Westby pointed me to this debian bug:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=322932

It seems that the easy fix is to just set 'GPG_TTY=$TTY'. I did some
testing on the command line, and that does seem to work.

# fails
echo 'foo' | gpg --cl
# succeeds
echo 'foo' | GPG_TTY=$TTY gpg --cl

So the attached patch basically just does this step. I want to include a
  NEWS entry about closing bug:
https://launchpad.net/products/bzr/+bug/54468

There is also this bug:
https://launchpad.net/products/bzr/+bug/44755
(gpg agent is not available, so gpg returns an exit code of '2' but
technically the request was properly signed because the user supplied
their passphrase).

Though I think that bug would have to wait until we get gpgme
integration. And we may not be able to fix it even then, since we want a
configurable gpg command (so that people can use custom agents). And I
don't know that gpgme works with generic gpg agent wrappers.

This fix does require users 'export TTY' in their shell startup scripts,
since that doesn't seem to be inherited by default.

John
=:->
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