Re: [PATCH v4 04/11] commit-reach(paint_down_to_common): prepare for handling shallow commits

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> When `git fetch --update-shallow` needs to test for commit ancestry, it
> can naturally run into a missing object (e.g. if it is a parent of a
> shallow commit). For the purpose of `--update-shallow`, this needs to be
> treated as if the child commit did not even have that parent, i.e. the
> commit history needs to be clamped.
>
> For all other scenarios, clamping the commit history is actually a bug,
> as it would hide repository corruption (for an analysis regarding
> shallow and partial clones, see the analysis further down).
>
> Add a flag to optionally ask the function to ignore missing commits, as
> `--update-shallow` needs it to, while detecting missing objects as a
> repository corruption error by default.
>
> This flag is needed, and cannot replaced by `is_repository_shallow()` to
> indicate that situation, because that function would return 0 in the
> `--update-shallow` scenario: There is not actually a `shallow` file in
> that scenario, as demonstrated e.g. by t5537.10 ("add new shallow root
> with receive.updateshallow on") and t5538.4 ("add new shallow root with
> receive.updateshallow on").