Fix git-am safety checks

Fix git-am safety checks

An earlier commit cbd64af added a check that prevents "git-am"
to run without its standard input connected to a terminal while
resuming operation.  This was to catch a user error to try
feeding a new patch from its standard input while recovery.

The assumption of the check was that it is an indication that a
new patch is being fed if the standard input is not connected to
a terminal.  It is however not quite correct (the standard input
can be /dev/null if the user knows the operation does not need
any input, for example).  This broke t3403 when the test was run
with its standard input connected to /dev/null.

When git-am is given an explicit command such as --skip, there
is no reason to insist that the standard input is a terminal; we
are not going to read a new patch anyway.

Credit goes to Gerrit Pape for noticing and reporting the
problem with t3403-rebase-skip test.

Signed-off-by: Junio C Hamano <junkio@cox.net>