git-rebase--interactive.sh: comparision with == is bashism

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -170,7 +170,7 @@ pick_one_preserving_merges () {
 
 	if test -f "$DOTEST"/current-commit
 	then
-		if [ "$fast_forward" == "t" ]
+		if test "$fast_forward" = t
 		then
 			cat "$DOTEST"/current-commit | while read current_commit
 			do