Fix URL parsing in IDE MRs

What does this MR do?

This fixed a regression introduced by moving the merge_requests route under the /-/ namespace. Currently we used the merge request URL to parse for the project's full path with namespace. Now, we are using the full reference.

This is just a quick fix, and the best fix would be to migrate this bit over to GraphQL so we don't have to worry about the issues that come with parsing strings for specific values. #201762

Screenshots

Before	After
ide_bug

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry Not needed since this is introduced and fixed in the same release.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

References

• Related to #200087 (closed)