When using `code-tools: source: repo` link to exact commit

Bug description

I think when linking to the source file, it would be nice if the code-tools: source: repo linked to exact commit for that file on GitHub.

For my personal blog, I used to do something like this:

const commit = child_process
  .spawnSync("git", ["log", "-n", "1", "--pretty=format:%H", "--", `${filename}`])
  .stdout.toString();
metadata.source = `https://github.com/kdheepak/blog/blob/${commit}/${filename}`;

Checklist

• Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
• Please format your issue so it is easier for us to read the bug report.
• Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
• Please document the operating system you're running. If on Linux, please provide the specific distribution.
• Please provide the output of quarto check so we know which version of quarto and its dependencies you're running.

Start by searching the code-tools implementation for the source: repo option and the metadata.source value shown in the issue. Trace how the source-file URL is generated, then verify that it points to the file at its exact Git commit on GitHub rather than a moving branch reference.