Fix notes when path names are the same

What does this MR do and why?

Issue #487375

This fixes a bug where referencing an epic in an issue created the wrong reference link when the group name has the same name as the project.

This was because the reference used the relative path, instead of the absolute path. E.g.

• Group name: gitlab
• Project name: gitlab (full path: /gitlab/gitlab)

When mentioning an epic within the gitlab/gitlab project, the note generated a reference to gitlab/gitlab/issues/:iid, instead of /gitlab/epics/:iid (the IID must exist in the project).

This is now fixed by using generating an absolute path as URL for the reference to mitigate the ambiguity.

References

Screenshots or screen recordings

Before	After
before_mentioning	after_mentioning

How to set up and validate locally

1. Create a group and project that have the same name, e.g. Group: gitlab, and project gitlab
2. Create an epic in the group
3. Create an issue in the group
4. Mention the issue in the epic with #:iid
5. Go to the mentioned issue, and check the system note that includes the mention
6. Before: It doesn't link to the epic but the issue again
7. After: It links to the epic

Same can be reproduced when mentioning a Merge Request in an Epic.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.