Permalinks in feature branches on forks should fallback to upstream main for unmodified lines

先检查与 permalink 相关的逻辑以及 issue 中提到的 build/filters.js 示例，然后查找涵盖 fork 和功能分支链接的测试。定义 upstream 默认分支或已配置分支的优先级，并验证未修改的行使用 upstream 链接，而修改过的行保留 fork 链接；issue 未指出任何现有测试或入口点。

Problem

Referencing prior fix in PR #1816: Permalinks generated from feature branches on forks do not fallback to upstream main, even for lines that are completely unmodified. This harms canonical link-sharing because:

1. Links to feature branches break when the branch is deleted (common after PR merge)
2. Recipients may not have access to fork repositories
3. The same code exists in upstream main but the link doesn't redirect there

Current Behavior

When creating a permalink from a feature branch on a fork:

• The link points to: fork/repo/blob/feature-branch/file.ext#L123
• No fallback occurs even if line 123 is identical in upstream main
• Link becomes invalid after branch deletion

Proposed Enhancement

Extend the permalink generation logic to:

1. Check if the referenced lines exist unchanged in upstream main
2. If unchanged, generate permalink to upstream main instead
3. Fall back to checking other long-lived branches if not in main
4. Only use feature branch permalink if code is actually modified

main here be substituted for any default branches, or custom list thereof user can config for reference in order of prefernce
ie main/master/release-5/release-4 whereby main is preferred, but if snippet only exists in a release branch, not main, release branch in upstream is still preferred vs fork link, since it affects ability to paste into upstream github and getting content previews.

ie. https://github.com/microsoft/vscode-pull-request-github/blob/45d42a8efb0d9ba5bbf9810dcb78ba865004f67c/build/filters.js#L14

vs

https://github.com/fork/vscode-pull-request-github/blob/45d42a8efb0d9ba5bbf9810dcb78ba865004f67c/build/filters.js#L14

Context Summary

Benefits

• More resilient permalink sharing
• Better canonical link behavior
• Reduced broken links after PR merges
• Improved accessibility for external collaborators

This would make permalinks more useful for documentation, issue references, and knowledge sharing.

related: https://github.com/microsoft/vscode-pull-request-github/issues/1781