fix(update): key update-check cache by current git head

[sunwz1115]

Summary

• Invalidate the update-check cache when the local git HEAD changes.
• Prevents stale behind counts after a local update/rebase in editable git checkouts.

Test Plan

• tests/hermes_cli/test_update_check.py

Uploaded from LangLang production local patch after rebasing on the latest origin/main.

[alt-glitch]

Duplicate of #9670 / #20792. Multiple open PRs already address update-check cache invalidation when local HEAD moves (also #18236, #21675). This is the same fix approach — keying cache by git HEAD.

[teknium1]

Thanks for the focused cache-invalidation fix. The on-disk stale-cache premise remains valid on current main: hermes_cli/banner.py:339-344 returns a fresh cache before local HEAD is examined.

Problems

• The PR's cache_rev is not None condition (hermes_cli/banner.py:274 on the PR head) disables cache hits for non-git/PyPI installs. Current main retains those hits and keys them by VERSION (hermes_cli/banner.py:330-364, commit 689ef5e233980f5d5a32080e959f44c8991dd03a).
• The PR predates that VERSION-keyed contract and removes ver; salvage should preserve it while adding the git-HEAD discriminator.
• tests/hermes_cli/test_update_check.py:89 covers legacy rev: null, but not a fresh git:oldhead entry whose current HEAD becomes newhead.
• Current _check_via_local_git() now probes remote get-url origin and shallow status (hermes_cli/banner.py:196,212), so the PR's narrower subprocess mocks need updating.

Suggested changes

• Retain ver == VERSION for all installs and require matching HEAD only for local git checkouts.
• Add the fresh old-HEAD/new-HEAD regression case and current git preflight mock responses.

Automated hermes-sweeper review.

[teknium1]

When neither HERMES_REVISION nor a local .git checkout exists, cache_rev stays None, so this rejects every otherwise-valid PyPI cache entry. Preserve current main's ver == VERSION cache path (689ef5e) and apply HEAD matching only to git installs.

[teknium1]

This verifies migration from the legacy rev: null schema, not the steady-state bug. Add a fresh rev: "git:oldhead" case with git rev-parse HEAD returning newhead, and assert it rechecks and rewrites the cache.

[teknium1]

When salvaging onto current main, this mock must also handle git remote get-url origin and git rev-parse --is-shallow-repository; current _check_via_local_git() runs both before fetch/rev-list.