fix: skip bogus shallow update counts

[supplefrog]

Summary

• make hermes update skip git rev-list --count when a shallow install has no visible merge-base with origin/<branch>
• compare tip SHAs in that case and print a generic update-available message instead of a bogus full-history count
• add regression coverage for the shallow/no-merge-base update path

Fixes #53479

Test Plan

• python -m pytest tests/hermes_cli/test_cmd_update.py::TestCmdUpdateBranchFallback::test_update_shallow_without_merge_base_does_not_count_history -q — passed
• python -m py_compile hermes_cli/main.py tests/hermes_cli/test_cmd_update.py — passed
• node --test apps/desktop/electron/update-count.test.cjs — passed

Notes

• python -m pytest tests/hermes_cli/test_cmd_update.py -q currently has Windows-local baseline failures unrelated to this change: one expectation assumes bare ['git'] while production uses ['git', '-c', 'windows.appendAtomically=false'], and one npm-call assertion does not observe npm calls in this Windows mocked run. The new regression test passes independently.

[alt-glitch]

This was generated by AI during triage.
Duplicate of #53494 (earliest open PR for #53479, created 08:20) — same mechanism: detect a shallow checkout with no merge-base via rev-parse --is-shallow-repository, skip git rev-list --count, and print a generic update-available message instead of a bogus count. #53498 is the sibling already linked to #53494. Marking duplicate so the maintainer can consolidate on the canonical PR.

[tonydwb]

Code Review Summary

Verdict: Comment

Fixes bogus shallow commit counts in cmd_update by detecting shallow repos and comparing tip SHAs instead of using rev-list --count. Includes a dedicated test.

Note: Competing fix pair detected — PRs #53498 and #53494 address the same issue. PR #53498 also adds --depth 1 for shallow fetches; PR #53494 extracts a shared _get_remote_update_commit_count() helper. All three implementations are independently sound. The maintainer should reconcile which approach to merge.

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Reviewed by Hermes Agent

[supplefrog]

Closing as duplicate. #53498 already implements the same fix for #53479 with broader targeted coverage and CI signal.