v1.87.7.0 fix(redact-prepush): four paths where the hook exits 0 on a real credential

Four paths made the pre-push credential scanner exit 0 with the secret going
out anyway, and two adjacent defects in the same functions had to land with
them.

Range resolution: defaultRemoteBranch() asked origin regardless of the push
target, so pushing to a second remote while HEAD matched origin/main resolved
HEAD..HEAD and scanned nothing; and a well-shaped but absent remote sha let a
guessed base's empty diff read as "nothing to scan". The probe is now scoped to
the push target and a guess that scanned nothing blocks with a fetch hint.

Slicing: the no-overlap argument holds for a pattern's match but not for its
proximity requirement, so a label at the end of one slice and its secret at the
start of the next never fired; and budgeting in raw bytes let zero-width
padding decide the seam using bytes the engine strips before matching. Slices
now overlap by 16 KiB and are budgeted in zero-width-stripped bytes.

Adjacent: the fallback range's hardcoded SHA-1 empty-tree id does not exist in
a SHA-256 repository and hard-blocked every legitimate first push there, which
the remote scoping makes reachable more often; and an over-budget single line
was handed to the engine whole, blocking without the content ever being read.

test/redact-prepush-fail-open.sh is the gate: 26 scenarios against real
repositories with synthetic credentials, PASS here and FAIL on the four rows
against the scanner this branch forks from.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

VERSION

@@ -1 +1 @@
-1.87.5.0
+1.87.7.0