fix(git-gate): scan $new --not --all for every push, not $old..$new

The pre-receive hook scanned existing-branch updates with the delta range
$old..$new. On a rebase / non-fast-forward force-push onto an advanced main,
$old is no longer an ancestor of $new, so $old..$new expands to all of main's
new history — including the deliberate sandbox-escape gitleaks fixtures — and
the push is rejected on commits that belong to main, not the branch.

Unify the range on `$new --not --all` for every non-delete push (this is the
deferred open question from PRD 0028, which already applied it to new refs
for #106). It scans only the commits the push introduces and is
security-equivalent: the bare repo's refs come only from trusted upstream
mirror-fetch and gitleaks-gated pushes, so an excluded commit is
already-upstream or already-scanned. It is also more correct for
non-fast-forward pushes, where $old..$new can skip commits off the direct path.

Fixes #421

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1qRZTJC6qgBsUSjNrBdkX

tests/unit/test_git_gate.py

@@ -168,16 +168,18 @@ class TestHookRender(unittest.TestCase):
         # Stdin is buffered to a tempfile so both phases can re-read.
         self.assertIn("refs_file=$(mktemp)", hook)
 
-    def test_new_ref_scan_scoped_to_incoming_commits(self):
-        # A new branch (old=all-zeros) must scan only commits new to the
-        # gate, not the full ancestry — otherwise historical findings
-        # block every new-branch push (PRD 0028 / issue #106).
+    def test_scan_scoped_to_incoming_commits(self):
+        # Every non-delete push scans only commits new to the gate, not
+        # the full ancestry and not the `$old..$new` delta — otherwise
+        # historical fixtures block new-branch pushes (PRD 0028 / #106)
+        # and a rebase/force-push onto an advanced main drags in main's
+        # history incl. the sandbox-escape fixtures (#346).
         hook = git_gate_render_hook()
         self.assertIn('log_opts="$new --not --all"', hook)
-        # The old over-broad full-ancestry range must be gone.
+        # Neither the full-ancestry range nor the ancestry-blind delta
+        # range may survive.
         self.assertNotIn('log_opts="$new"', hook)
-        # Existing-branch delta scan is unchanged.
-        self.assertIn('log_opts="$old..$new"', hook)
+        self.assertNotIn('log_opts="$old..$new"', hook)
 
     def test_forward_ssh_is_non_interactive_and_bounded(self):
         # No prompt (BatchMode) and a connect timeout, so an unreachable