am: drop tty requirement for --interactive

am: drop tty requirement for --interactive

We have required that the stdin of "am --interactive" be a tty since
a1451104ac (git-am: interactive should fail gracefully., 2005-10-12).
However, this isn't strictly necessary, and makes the tool harder to
test (and is unlike all of our other --interactive commands).

The goal of that commit was to make sure that somebody does not do:

  git am --interactive <mbox

and cause us to read commands from the mbox. But we can simply check
up front for this case and complain before entering the interactive
loop.

Technically this disallows:

  git am --interactive </dev/null

where our lack of patches means we would never prompt for anything, and
so the old code would not notice our lack of tty (and now we'd die
early). But since such a command is totally pointless, it's no loss.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>