git-am.sh: fix initialization of the threeway variable

Initialization for the threeway variable was missing. This caused
a behavior change for command lines like:

	threeway=t git am ...

This commit adds initialization for this variable.
Signed-off-by: NRemi Lespinet <remi.lespinet@ensimag.grenoble-inp.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-am.sh

@@ -378,6 +378,7 @@ committer_date_is_author_date=
 ignore_date=
 allow_rerere_autoupdate=
 gpg_sign_opt=
+threeway=
 
 if test "$(git config --bool --get am.messageid)" = true
 then