builtin-am: reject patches when there's a session in progress

builtin-am: reject patches when there's a session in progress

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
would error out if the user gave it mbox(s) on the command-line, but
there was a session in progress.

Since c95b138 (Fix git-am safety checks, 2006-09-15), git-am would
detect if the user attempted to feed it a mbox via stdin, by checking if
stdin is not a tty and there is no resume command given.

Re-implement the above two safety checks.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>