am: read interactive input from stdin

am: read interactive input from stdin

In the conversion of git-am from shell script to C, we switched to using
git_prompt(). Unlike the original shell command "read reply", this
doesn't read from stdin at all, but rather from /dev/tty.

In most cases this distinction wouldn't matter. We require (as the shell
script did) that stdin is a tty, so they would generally be the same
thing. But one important exception is our test suite: even with
test_terminal, we cannot test "am --interactive" because it insists on
reading from /dev/tty, not the pseudo-tty we've set up in the test
script.

Fixing this clears the way to adding tests in a future patch.

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