fix(content-docs): read last update from inner git repositories

[felipecrs]

This reapplies #5048, because it somehow got reverted during a refactoring (I believe).

To prevent it from happening again in future, I added a comment in the code.

This also:

• Expand -1 into --max-count=1 (long options are always preferred for better readability)

• Add -- to prevent file names who looks like git options to clash

• Check if the file exists before issuing git and return a proper error message

  • git log is known to return exit code 0 but no output in some cases, such as when using -- and the file does not exist
  • If preferred, I can instead check if stdout is empty rather than proactively checking the file

[netlify]

✔️ [V2]
Built without sensitive environment variables

🔨 Explore the source changes: 5612b60

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61fb5241e206bf0007da92c0

😎 Browse the preview: https://deploy-preview-6592--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	90
🟢 Accessibility	100
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	92

Lighthouse ran on https://deploy-preview-6592--docusaurus-2.netlify.app/

[felipecrs]

One check cancelled? Should I do something?

[Josh-Cena]

Thanks! It makes sense to me