t3404: add failing branch symref test

t3404: add failing branch symref test

rebase --update-refs queues local branch decorations by their literal
refnames. When a branch such as refs/heads/main is a symbolic ref to
the current branch, the normal rebase path first updates the current
branch and the queued symref update later tries to update the same
referent with the old value it recorded before the rebase.

Add a known-breakage test that exercises this case so that the fix can
flip it to test_expect_success. The expected behavior is that the branch
symref keeps pointing at the rebased current branch.

Signed-off-by: Son Luong Ngoc <sluongng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>