am/apply: warn if we end up reading patches from terminal

am/apply: warn if we end up reading patches from terminal

In an interactive session, "git am" without arguments, or even
worse, "git am --whitespace file", waits silently for the user to
feed the patches from the standard input (presumably by typing or
copy-pasting).  Give a feedback message to the user when this
happens, as it is unlikely that the user meant to do so.

Signed-off-by: Junio C Hamano <gitster@pobox.com>