Re: t9210-scalar.sh fails with SANITIZE=undefined

> Running t9210 with the tip of master triggers a problem with UBSan:
> 
>   $ make SANITIZE=undefined
>   [...]
>   $ cd t
>   $ ./t9210-scalar.sh -v -i
>   [...]
>   read-cache.c:1886:46: runtime error: member access within misaligned address 0x7f7c09bf7055 for type 'struct ondisk_cache_entry', which requires 4 byte alignment
>   0x7f7c09bf7055: note: pointer points here
>   33 2e 74 00 63 2c 31  42 17 3f 49 72 63 2c 31  42 17 3f 49 72 00 00 fe  01 02 60 06 4d 00 00 81  a4
>               ^
> 
> Now here's the interesting part. We do carefully read most of the data
> out of the struct with get_be16(), which should handle alignment (we
> have to do so because that on_disk_cache_entry is literally just a cast
> from an mmap'd buffer). But the line in question is just:
> 
>   const uint16_t *flagsp = (const uint16_t *)(ondisk->data + hashsz);
> 
> It's not even reading anything, but just computing an offset within the
> struct. I don't think that line in particular is to blame. If I use an
> older version of Git that predates it on the same repo generated by
> t9210, I get a similar error for a different line.
> 
> Another thing to note is that the command which fails isn't scalar
> itself! It's just vanilla "git add -- loose.t". But it's curious that we
> never saw this alignment problem before. I wonder if the scalar-cloned
> repository has some index options turned on that trigger the issue.
> 
> I didn't dig further. It's obviously new in v2.38.0-rc1, but I'm not
> sure it's a show-stopper. It _might_ have been there all along, and is
> just now surfacing. Or it might be in an existing experimental feature
> that just wasn't exercised enough in the tests. Or if it really is new
> in scalar, then it will only hurt people using scalar, which didn't
> exist before. So I don't think it's a regression in the strictest sense,
> but it might be nice to get a more accurate understanding of the problem
> before the release.