rebase: honor --rerere-autoupdate

Rebase accepts '--rerere-autoupdate' as an option but only honors it
if '-m' is also given. Fix it for a non-interactive rebase by passing
on the option to 'git am' and 'git cherry-pick'. Rework the tests so
that they can be used for each rebase flavor and extend them.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--am.sh

@@ -45,7 +45,7 @@ then
 	# itself well to recording empty patches.  fortunately, cherry-pick
 	# makes this easy
 	git cherry-pick ${gpg_sign_opt:+"$gpg_sign_opt"} --allow-empty \
-		--right-only "$revisions" \
+		$allow_rerere_autoupdate --right-only "$revisions" \
 		${restrict_revision+^$restrict_revision}
 	ret=$?
 else
@@ -82,6 +82,7 @@ else
 	fi
 
 	git am $git_am_opt --rebasing --resolvemsg="$resolvemsg" \
+		$allow_rerere_autoupdate \
 		${gpg_sign_opt:+"$gpg_sign_opt"} <"$GIT_DIR/rebased-patches"
 	ret=$?