git-am expects the variable $resume to be empty or unset, which might not
be the case if $resume is set in the user's environment. So initialize
it to an empty value on startup.
The problem was noticed by Pierre Habouzit and reported through
http://bugs.debian.org/435807
Signed-off-by: Gerrit Pape <pape@smarden.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>