t9210-scalar.sh fails with SANITIZE=undefined

[Jeff King <peff@peff.net>]

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.

-Peff