From: Johannes Schindelin <johannes.schindelin@gmx.de>

When rearranging the todo list so that the fixups/squashes are reordered
just after the commits they intend to fix up, we use two arrays to
maintain that list: `next` and `tail`.

The idea is that `next[i]`, if set to a non-negative value, contains the
index of the item that should be rearranged just after the `i`th item.

To avoid having to walk the entire `next` chain when appending another
fixup/squash, we also store the end of the `next` chain in `tail[i]`.

The logic we currently use to update these array items is based on the
assumption that given a fixup/squash item at index `i`, we just found
the index `i2` indicating the first item in that fixup chain.

However, as reported by Paul Ganssle, that need not be true: the special
form `fixup! <commit-hash>` is allowed to point to _another_ fixup
commit in the middle of the fixup chain.

Example:

	* 0192a To fixup
	* 02f12 fixup! To fixup
	* 03763 fixup! To fixup
	* 04ecb fixup! 02f12

Note how the fourth commit targets the second commit, which is already a
fixup that targets the first commit.

The good news is that it is easy to fix this: we use the correct
condition (we now possibly set `tail[i2]` even for fixups in the middle)
and we _also_ have to ensure that we _insert_ the item rather than
_append_ it, i.e. we need to set `next[i2]` accordingly (it might still
be set to `-1` if it was actually appended).

Reported-by: Paul Ganssle <paul@ganssle.io>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Submitted-As: https://lore.kernel.org/git/pull.625.v2.git.1588717980225.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.625.git.1588624804554.gitgitgadget@gmail.com