From: John Cai <johncai86@gmail.com>

Fixes a bug whereby rebase updates the deferenced reference HEAD points
to instead of HEAD directly.

If HEAD is on main and if the following is a fast-forward operation,

git rebase $(git rev-parse main) $(git rev-parse topic)

Instead of HEAD being set to $(git rev-parse topic), rebase erroneously
dereferences HEAD and sets main to $(git rev-parse topic). This bug was
reported by Michael McClimon. See [1].

This is happening because on a fast foward with an oid as a <branch>,
update_refs() will only call update_ref() with REF_NO_DEREF if
RESET_HEAD_DETACH is set. This change was made in 176f5d96 (built-in rebase
--autostash: leave the current branch alone if possible,
2018-11-07). In rebase, we are not setting the RESET_HEAD_DETACH flag,
which means that the update_ref() call ends up dereferencing
HEAD and updating it to the oid used as <branch>.

The correct behavior is that git rebase should update HEAD to $(git
rev-parse topic) without dereferencing it.

Fix this bug by adding the RESET_HEAD_DETACH flag in checkout_up_to_date
so that once reset_head() calls update_refs(), it calls update_ref() with
REF_NO_DEREF which updates HEAD directly intead of deferencing it and
updating the branch that HEAD points to.

Also add a test to ensure this behavior.

1. https://lore.kernel.org/git/xmqqsfrpbepd.fsf@gitster.g/

Signed-off-by: John Cai <johncai86@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1226.git.git.1646975144178.gitgitgadget@gmail.com