Re: [PATCH 2/2] repack -ad: prune the list of shallow commits

>
> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> While it is true that we never add unreachable commits into pack files
> intentionally (as `git repack`'s documentation states), we must not
> forget that a `git fetch --prune` (or even a `git fetch` when a ref was
> force-pushed in the meantime) can make a commit unreachable that was
> reachable before.
>
> Therefore it is not safe to assume that a `git repack -adlf` will keep
> unreachable commits alone (under the assumption that they had not been
> packed in the first place).
>
> This is particularly important to keep in mind when looking at the
> `.git/shallow` file: if any commits listed in that file become
> unreachable, it is not a problem, but if they go missing, it *is* a
> problem. One symptom of this problem is that a deepening fetch may now
> fail with
>
>         fatal: error in object: unshallow <commit-hash>
>