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>