git-am: initialize variable $resume on startup

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>

git-am.sh

@@ -103,7 +103,8 @@ It does not apply to blobs recorded in its index."
 }
 
 prec=4
-dotest=.dotest sign= utf8=t keep= skip= interactive= resolved= binary= resolvemsg=
+dotest=.dotest sign= utf8=t keep= skip= interactive= resolved= binary=
+resolvemsg= resume=
 git_apply_opt=
 
 while case "$#" in 0) break;; esac