Re: [PATCH v5 0/5] Fix use of uninitialized hash algorithms

> A change recently merged to 'next' stops us from defaulting to using
> SHA-1 unless other code (like a logic early in the start-up sequence
> to see what hash is being used in the repository we are working in)
> explicitly sets it, leading to a (deliberate) crash of "git" when we
> forgot to cover certain code paths.
> 
> It turns out we have a few.  Notable ones are all operations that
> are designed to work outside a repository.  We should go over all
> such code paths and give them a reasonable default when there is one
> available (e.g. for historical reasons, patch-id is documented to
> work with SHA-1 hashes, so arguably it, or at least when it is
> invoked with the "--stable" option, should do so everywhere, not
> just in SHA-1 repositories, but in SHA-256 repository or outside any
> repository).  In the meantime, if an end-user hits such a "bug"
> before we can fix it, it would be nice to give them an escape hatch
> to restore the historical behaviour of falling back to use SHA-1.
> 
> These patches are designed to apply on a merge of c8aed5e8
> (repository: stop setting SHA1 as the default object hash,
> 2024-05-07) into 3e4a232f (The third batch, 2024-05-13), which has
> been the same base throughout the past iterations.
> 
> In this fifth iteration:
> 
>  - The first step no longer falls back to GIT_DEFAULT_HASH; the
>    escape hatch is a dedicated GIT_TEST_DEFAULT_HASH_ALGO
>    environment variable, but hopefully we do not have to advertise
>    it all that often.
> 
>  - The second step has been simplified somewhat to use the "nongit"
>    helper when we only need to run a single "git" command in t1517.
>    The way the expected output files were prepared in the previous
>    versions did not correctly force use of SHA-1 algorithm, which
>    has been corrected.  The third step and fourth step for t1517
>    continue to be "flip expect_failure to expect_success", but you
>    can see context differences in the range-diff.
> 
>  - The fourth step also has a fix for t1007 where the previous
>    iterations did not correctly force use of SHA-1 to prepare the
>    expected output.
> 
> Otherwise this round should be ready, modulo possible typoes.