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>