t5520: test --rebase failure on unborn branch with index

t5520: test --rebase failure on unborn branch with index

Commit 19a7fcb (allow pull --rebase on branch yet to be born,
2009-08-11) special cases git-pull on an unborn branch in a different
code path such that git-pull --rebase is still valid even though there
is no HEAD yet.

This code path still ensures that there is no index in order not to lose
any staged changes. Implement a test to ensure that this check is
triggered.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>