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>