GetPatchID computes a patch ID by piping git-diff(1) into
`git patch-id --verbatim`. We use `--verbatim` to keep whitespace, but it
also turns on `--stable`, which hashes each file diff on its own and adds
the hashes together. That sum only makes the ID independent of the order
the files appear in, and we do not need it: Gitaly generates the diff
itself, so the order is always the same.

`--unstable` hashes the whole diff at once, which is what we want, but git
will not combine it with `--verbatim`. So this adds a whole_diff_hash
option that skips git-patch-id(1) altogether. When it is set, Gitaly
normalizes the diff exactly as `git patch-id --verbatim` would and hashes
it once with SHA256. Hunk headers and "index" lines are left out so that
line numbers and blob IDs do not change the result, which is what lets a
patch ID survive a rebase.

Our normalization matches git's in every case but one: we also hash the
file header that ends a binary diff, which git drops. This changes
nothing that exists today, since whole_diff_hash is new, and it only
matters in a rare case, a mode-only change right after a binary diff.
There git hashes nothing that identifies the file, so two patches that
change the same binary but chmod different files get the same ID. Hashing
the header keeps them apart.

Gitaly always passes --full-index, so a binary diff should always carry
an "index" line. A missing one is treated as an error only as a safety
check.

The diff is read with ReadSlice(3) instead of ReadBytes(3) so we do not
allocate a string for every line. On a ~40MB diff this roughly halves the
time and cuts allocations from about 205k to 4.

The normalization is tested against git itself, not against our reading
of git. For a single-file patch git's per-file sum equals a single hash,
so `git patch-id --verbatim` is an exact reference. Multi-file patches are
checked by splitting the diff per file and reproducing git's sum.

Fixes: https://gitlab.com/gitlab-org/gitaly/-/issues/7350
Changelog: added
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Add whole-diff hash option to GetPatchID

See merge request gitlab-org/gitaly!9170

Merged-by: Olivier Campeau <ocampeau@gitlab.com>
Approved-by: Olivier Campeau <ocampeau@gitlab.com>
Co-authored-by: Mustafa Bayar <mbayar@gitlab.com>

[ci skip]