unseen: revert the batched-blob materialize (it failed git fetch on ~half of repos)

Git.materialize enumerated every object reachable from HEAD and tried to
`git fetch --no-filter` them by SHA. Git rejects that for real repos
("did not send all necessary objects" / "bad revision"), exiting 1 ->
InstrumentFailure(git_exit_1) that failed the whole repository. Measured
in a live draw: 326/670 attempts, ~49% of all failures, which froze the
accepted pool at 44/300. Reproduced on pallets/flask (26221 reachable,
9287 missing, fetch exit 1).

Git cannot batch-fetch arbitrary tree/blob objects by SHA, so remove
materialize and restore the original lazy path: the blob:none clone keeps
the commit graph and the introducing/blame/ordinary-history reads fetch
their blobs on demand through Git's promisor. The per-fix tolerant fetch
loop, the process-pool parallelism, the 25-with-zero-entries guard, the
per-repo deadline and resume are unchanged. Extraction content is
byte-identical (only the prefetch is gone).

217 unseen tests pass, ruff clean.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

benchmarks/unseen/draft.py

@@ -154,7 +154,7 @@ class Clones:
             git = Git(path, runner=self.runner, limit_bytes=self.limit_bytes, deadline_seconds=self.deadline_seconds)
             try:
                 # Full commit graph is needed for temporal thirds and OSV brackets.
-                # repository_changes batches blobs before reading their content.
+                # Content reads fetch blobs on demand through Git's promisor mechanism.
                 done = self.runner(
                     ["git", "-c", "credential.helper=", "clone", "--quiet", "--filter=blob:none", "--no-checkout", url, str(path)],
                     stdin=subprocess.DEVNULL,
@@ -222,8 +222,6 @@ def repository_changes(git, candidate, fixes: set[str], *, seed: int):
         except InstrumentFailure as exc:
             if fix == candidate.fix or not str(exc).startswith("git_exit_"):
                 raise
-    # Batch only resolved histories before diff/show/blame can fetch per-file blobs.
-    git.materialize(["HEAD", *resolved])
     proof = prove_source(git, candidate.parent)
     change = extract.introducing(git, candidate)
     known = set()