[BUG]: ComputeChangedFiles and ComputeAffectedTests reflect non-PR changes from merged base branch

[Rd4dev]

Describe the bug

In the current workflow, ComputeChangedFiles and ComputeAffectedTests functions may inaccurately include changes from the base branch when a merge is performed before the latest commit. This behavior results in additional files being referenced as "changed," even though they are not directly modified in the current PR.

However, once a subsequent commit is pushed to the PR, this issue typically resolves, and only the actual changes in the PR are reflected.

This issue affects the code coverage report by unnecessarily including non-PR changes in the list of changed files and reporting them in the coverage results after a merge with the base branch. As shown in this coverage report, even files unrelated to the PR (such as a wiki page with no Kotlin file changes) can appear with coverage data simply due to changes from the base branch merge.

Steps To Reproduce

1. Create a feature branch from the main branch.
2. Make a few commits with changes specific to this feature branch.
3. Merge the latest changes from the main branch into the feature branch.
4. View the Unit Test's Compute affected tests - Compute test matrix or Code Coverage's Compute changed files - Compute file matrix to find non-PR changes included from the merge.

Expected Behavior

The ComputeChangedFiles and ComputeAffectedTests retrievals should list only the files that have been changed or affected by the PR commits and exclude any unrelated changes from the merge with the base branch.

Screenshots/Videos

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What device/emulator are you using?

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Which Android version is your device/emulator running?

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Which version of the Oppia Android app are you using?

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Additional Context

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