rebase-interactive: simplify pick_on_preserving_merges

Use compound if statement instead of nested if statements to
simplify pick_on_preserving_merges.

Signed-off-by: Wink Saville <wink@saville.com>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -307,9 +307,7 @@ pick_one_preserving_merges () {
 	esac
 	sha1=$(git rev-parse $sha1)
 
-	if test -f "$state_dir"/current-commit
-	then
-		if test "$fast_forward" = t
+	if test -f "$state_dir"/current-commit && test "$fast_forward" = t
 	then
 		while read current_commit
 		do
@@ -318,7 +316,6 @@ pick_one_preserving_merges () {
 		rm "$state_dir"/current-commit ||
 			die "$(gettext "Cannot write current commit's replacement sha1")"
 	fi
-	fi
 
 	echo $sha1 >> "$state_dir"/current-commit