Fix git-am safety checks

An earlier commit cbd64afb 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: NJunio C Hamano <junkio@cox.net>

git-am.sh

@@ -166,10 +166,25 @@ fi
 
 if test -d "$dotest"
 then
-	if test ",$#," != ",0," || ! tty -s
-	then
+	case "$#,$skip$resolved" in
+	0,*t*)
+		# Explicit resume command and we do not have file, so
+		# we are happy.
+		: ;;
+	0,)
+		# No file input but without resume parameters; catch
+		# user error to feed us a patch from standard input
+		# when there is already .dotest.  This is somewhat
+		# unreliable -- stdin could be /dev/null for example
+		# and the caller did not intend to feed us a patch but
+		# wanted to continue unattended.
+		tty -s
+		;;
+	*)
+		false
+		;;
+	esac ||
 	die "previous dotest directory $dotest still exists but mbox given."
-	fi
 	resume=yes
 else
 	# Make sure we are not given --skip nor --resolved