fix: Use absolute (relative to repo root) path to supplied filename

[mattcen]

If a file is referenced from the subdir of a repository, the script confirms it exists and tacks the filename onto the URL, but doesn't check if the filename provided is relative to the repo root.

I couldn't immediately work out the best way to update the tests to:

1. Create a temp dir inside the existing temp repo
2. Create a new temp file within that new dir
3. Change CWD to the new dir
4. Run git-browse on that new file with a relative path