fix(stack): anchor old PR head SHAs in local refs for revision history

[jd]

fetch_old_pr_heads previously fetched refs/pull/N/head with no
destination refspec, leaving the SHA reachable only via FETCH_HEAD —
which any subsequent fetch (or background gc) can drop. When the
SHA is no longer locally reachable, git show <old_sha> fails inside
_git_patch_id, and detect_change_type falls back to the
"unknown" classification in the PR's revision-history comment.

Fetch into a stable per-PR ref under refs/mergify/stack/old-pr-heads/<N>
(forced refspec, with --no-write-fetch-head to avoid touching the
shared FETCH_HEAD pointer). The SHA is then anchored by a real ref
for the lifetime of the patch-id comparison and beyond, so future
runs can compare even after a long gap.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	fix(stack): anchor old PR head SHAs in local refs for revision history	#1314	👈
2	fix(stack): surface fetch_old_pr_heads failures instead of swallowing	#1315

[mergify]

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[Copilot]

Pull request overview

This PR fixes revision-history change-type detection in stack push by ensuring “old PR head” SHAs remain locally reachable (not just via FETCH_HEAD) so patch-id comparisons don’t degrade to "unknown" after subsequent fetches or garbage collection.

Changes:

• Fetch refs/pull/<N>/head into stable local refs under refs/mergify/stack/old-pr-heads/<N> using a forced refspec.
• Add --no-write-fetch-head to avoid mutating the shared FETCH_HEAD.
• Update stack push tests to assert the new git fetch invocation/refspecs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
mergify_cli/stack/push.py	Implements anchored per-PR local refs for old head SHAs and avoids writing FETCH_HEAD.
mergify_cli/tests/stack/test_push.py	Updates mocks/assertions to match the new fetch refspec + flags.

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