rebase -i: don't error out if $state_dir already exists

rebase -i: don't error out if $state_dir already exists

In preparation for a later patch that will create $state_dir/autostash
in git-rebase.sh before anything else can happen, change a `mkdir
$state_dir` call to `mkdir -p $state_dir`.  The change is safe,
because this is not a test to detect an in-progress rebase (that is
already done much earlier in git-rebase.sh).

Signed-off-by: Ramkumar Ramachandra <redacted>
Signed-off-by: Junio C Hamano <redacted>