rebase -i: don't read unused variable preserve_merges

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>