rebase -i: fix for optional [branch] parameter

When calling "git rebase -i <upstream> <branch>", git should switch
to <branch> first.  This worked before, but I broke it by my
"Shut git rebase -i up" patch.

Fix that, and add a test to make sure that it does not break again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -405,6 +405,7 @@ do
 
 		require_clean_work_tree
 
+		mkdir "$DOTEST" || die "Could not create temporary $DOTEST"
 		if test ! -z "$2"
 		then
 			output git show-ref --verify --quiet "refs/heads/$2" ||
@@ -418,7 +419,6 @@ do
 
 		test -z "$ONTO" && ONTO=$UPSTREAM
 
-		mkdir "$DOTEST" || die "Could not create temporary $DOTEST"
 		: > "$DOTEST"/interactive || die "Could not mark as interactive"
 		git symbolic-ref HEAD > "$DOTEST"/head-name ||
 			die "Could not get HEAD"

t/t3404-rebase-interactive.sh

@@ -98,6 +98,14 @@ test_expect_success 'no changes are a nop' '
 	test $(git rev-parse I) = $(git rev-parse HEAD)
 '
 
+test_expect_success 'test the [branch] option' '
+	git checkout -b dead-end &&
+	git rm file6 &&
+	git commit -m "stop here" &&
+	git rebase -i F branch2 &&
+	test $(git rev-parse I) = $(git rev-parse HEAD)
+'
+
 test_expect_success 'rebase on top of a non-conflicting commit' '
 	git checkout branch1 &&
 	git tag original-branch1 &&