Pipeline Graph: Improve recursive ancestoring

What does this MR do?

Adds earlier deduplication calls in the ancestor graph creation to help address #339259 (closed).

While there are broader fixes to pursue, I hope this will address the immediate issue.

Screenshots or Screencasts (strongly suggested)

matrix table	other links

How to setup and validate locally (strongly suggested)

1. Set up a Runner for your GDK.
2. Add a moderately-complex CI file that uses matrices. Example in #339259 (closed).
3. Add a CI file that uses needs, to be certain we are not breaking non-parallel options.
4. View the pipeline graph page for each (accessible from the pipelines list).

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Related to #339259 (closed)