minimize lenth of path to gpg agent socket

minimize lenth of path to gpg agent socket

Considered using the system tmp dir rather than putting it inside .t/,
but then if TEMP were set to a long path, that would be a problem.
Relative path seems the best approach, and will always be nice and
short.

The only downside of it is, if git-annex somehow changes the cwd
while running, it would break. But git-annex does not do that, and
should never do that.