fix(cli): avoid bogus shallow update counts

[LeonSGP43]

Summary

• avoid exact rev-list counts when a shallow checkout has no merge-base with the remote branch
• reuse the same guarded count helper in both hermes update and hermes update --check
• add CLI tests that assert shallow/no-merge-base repos skip rev-list --count and show a generic update message

Testing

• uv run python --version
• git diff --check
• uv run --extra dev python -m pytest tests/hermes_cli/test_cmd_update.py -k "shallow_history or check_reports_generic_update_when_shallow_history_has_no_merge_base"
• uv run --extra dev python -m pytest tests/hermes_cli/test_cmd_update.py
• uv run --extra dev ruff check hermes_cli/main.py tests/hermes_cli/test_cmd_update.py

[tonydwb]

Code Review Summary

Verdict: Comment

Addresses the same shallow clone bogus commit count issue as #53512 and #53498. Extracts a shared _get_remote_update_commit_count() helper that handles both shallow and non-shallow repos, and applies it to both cmd_update and cmd_update --check.

Note: Competing fix pair with #53512 and #53498. This is the most comprehensive of the three — it also fixes the --check path and extracts a reusable helper. The maintainer should reconcile which approach to merge.

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Reviewed by Hermes Agent

[teknium1]

Thanks for covering both updater entry points and adding regression intent. The apply-path issue is still present on current main, but this patch needs a re-scope before it can fix it safely.

Problems

• hermes_cli/main.py:9595-9600 on current main fetches before the PR's proposed count helper would run. Merged #50784 (eecb5b9dd19a4234ebf64c45e5440d85c60a6696) establishes that shallow detection must occur before fetch and that shallow fetches need --depth 1; otherwise the checkout can be unshallowed before the guard evaluates it.
• The --check portion is already implemented on main: hermes_cli/main.py:8486-8590 detects shallow state pre-fetch, preserves depth, compares tip SHAs, and skips rev-list.

Suggested changes

• Salvage the apply-path fix by moving the shallow probe ahead of hermes_cli/main.py:9595 and using a depth-preserving fetch plus SHA presence check there.
• Drop the duplicate --check changes and rebase the tests on the current updater layout.

Automated hermes-sweeper review.

[teknium1]

This shallow probe is reached only after _cmd_update_impl has fetched from origin. The existing merged fix #50784 requires probing before fetch and using --depth 1; otherwise a plain fetch can unshallow the checkout before this guard runs, so the exact-count path remains reachable for the reported failure.

[teknium1]

Closing — resolved by PR #86257 + #86318 (merged). Your _get_remote_update_commit_count design (shallow + no merge-base ⇒ generic update message, exact count otherwise) matches what landed for the --check path; the merged version extends it with exact-count recovery via the GitHub compare API so shallow installs usually still get a real number. Your branch targeted main.py before the update code moved to update_cmd.py, so the commit no longer applied directly — but you're credited in #86318's description alongside #53498. Thanks!