Fail for references to files outside of the repo

[dcastro]

Clarification and motivation

Given the following file structure:

- /dev/
  - repo1/
    - a.md
    - b.md
  - repo2/
    - c.md

Say the file a.md in repo1 contains a reference to a file in repo2, e.g. [some text](../repo2/c.md).

At the moment, if I run xrefcheck in repo1, it'll succeed - but it shouldn't.

The link in a.md points to a file outside of the repo's root (/dev/repo1). When the repo is pushed to github, and the markdown file is rendered by github, the link will be broken.

As such, xrefcheck should report this link as broken.

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Additionally, pretend a.md had a link such as ../repo1/c.md.

Locally, that path does point to a file inside the repo root. However, that won't work when the markdown file is rendered by github.

We should report this type of links as broken as well.

Same for ../repo2/../repo1/c.md and similar shenanigans.

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Note: the error message should be very clear about why this link is being reported as broken.

Acceptance criteria

• Links that point to files outside the repo are reported as broken.