rebase: set REF_HEAD_DETACH in checkout_up_to_date()

"git rebase A B" where B is not a commit should behave as if the
HEAD got detached at B and then the detached HEAD got rebased on top
of A.  A bug however overwrites the current branch to point at B,
when B is a descendant of A (i.e. the rebase ends up being a
fast-forward).  See [1] for the original bug report.

The callstack from checkout_up_to_date() is the following:

cmd_rebase()
-> checkout_up_to_date()
   -> reset_head()
      -> update_refs()
         -> update_ref()

When B is not a valid branch but an oid, rebase sets the head_name
of rebase_options to NULL. This value gets passed down this call
chain through the branch member of reset_head_opts also getting set
to NULL all the way to update_refs().

Then update_refs() checks ropts.branch to decide whether or not to switch
branches. If ropts.branch is NULL, it calls update_ref() to update HEAD.
At this point however, from rebase's point of view, we want a detached
HEAD. But, since checkout_up_to_date() does not set the RESET_HEAD_DETACH
flag, the update_ref() call will deference HEAD and update the branch its
pointing to. We want the HEAD detached at B instead.

Fix this bug by adding the RESET_HEAD_DETACH flag in
checkout_up_to_date if B is not a valid branch, 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 the correct behavior.

[1] https://lore.kernel.org/git/YiokTm3GxIZQQUow@newk/

Reported-by: Michael McClimon <michael@mcclimon.org>
Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: john-cai

builtin/rebase.c

@@ -829,6 +829,8 @@ static int checkout_up_to_date(struct rebase_options *options)
 	ropts.oid = &options->orig_head;
 	ropts.branch = options->head_name;
 	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
+	if (!ropts.branch)
+		ropts.flags |=  RESET_HEAD_DETACH;
 	ropts.head_msg = buf.buf;
 	if (reset_head(the_repository, &ropts) < 0)
 		ret = error(_("could not switch to %s"), options->switch_to);

t/t3400-rebase.sh

@@ -394,6 +394,15 @@ test_expect_success 'switch to branch not checked out' '
 	git rebase main other
 '
 
+test_expect_success 'switch to non-branch detaches HEAD' '
+	git checkout main &&
+	old_main=$(git rev-parse HEAD) &&
+	git rebase First Second^0 &&
+	test_cmp_rev HEAD Second &&
+	test_cmp_rev main $old_main &&
+	test_must_fail git symbolic-ref HEAD
+'
+
 test_expect_success 'refuse to switch to branch checked out elsewhere' '
 	git checkout main &&
 	git worktree add wt &&