git am: Remove stray error message from sed

When --continue is invoked without any changes, the following stray
error message appears- sed: can't read $dotest/final-commit: No such
file or directory. Remove this by making sure that the file actually
exists.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -693,7 +693,13 @@ do
 	else
 	    action=yes
 	fi
-	FIRSTLINE=$(sed 1q "$dotest/final-commit")
+
+	if test -f "$dotest/final-commit"
+	then
+		FIRSTLINE=$(sed 1q "$dotest/final-commit")
+	else
+		FIRSTLINE=""
+	fi
 
 	if test $action = skip
 	then