rebase -i: don't read unused variable preserve_merges

Since 8e4a91b (rebase -i: remember the settings of -v, -s and -p when
interrupted, 2007-07-08), the variable preserve_merges (then called
PRESERVE_MERGES) was detected from the state saved in
$GIT_DIR/rebase-merge in order to be used when the rebase resumed, but
its value was never actually used. The variable's value was only used
when the rebase was initated.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -583,7 +583,6 @@ skip_unnecessary_picks () {
 }
 
 get_saved_options () {
-	test -d "$rewritten" && preserve_merges=t
 	test -f "$state_dir"/rebase-root && rebase_root=t
 }