fix(internal/gapicgen): handle deleted files in git history

[quartzmo]

The genbot would previously crash if it encountered a commit that deleted a proto file. This was because it was trying to read the file from the current filesystem, where it no longer existed.

This change updates the genbot to read the file content from the historical commit directly, making the process resilient to file deletions and renames.i

refs: #13188
refs: #13189

[gemini-code-assist]

Summary of Changes

Hello @quartzmo, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the genbot's ability to process Git history by addressing a critical vulnerability where it would fail upon encountering deleted or renamed proto files. By introducing a method to retrieve file content directly from historical commits, the bot can now accurately analyze changes without relying on the current filesystem state, thereby ensuring more reliable and resilient code generation workflows.

Highlights

• Genbot Crash Fix: Resolved an issue where the genbot would crash when encountering deleted proto files in Git history.
• Historical Content Retrieval: Implemented a mechanism to read file content directly from specific historical Git commits.
• Resilience Improvement: Enhanced the genbot's robustness to handle file deletions and renames gracefully.

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[gemini-code-assist]

Code Review

This pull request effectively addresses a crash that occurred when the genbot processed a commit involving a deleted proto file. The approach of reading file content from the git history is sound, and the refactoring of goPkg into parseGoPkg improves code modularity. I have one suggestion to enhance the error logging for better debuggability. Overall, this is a solid improvement.

[bhshkh]

Please resolve vet failures.

[quartzmo]

@bhshkh Sorry, vet was broken by the Gemini reviewer's suggested commit! Fixed now.