fix: review request placeholder files

[alexanderleegs]

This PR fixes an issue where compareDiff was not filtering placeholder files. This affected any review requests which involved placeholder files, as our frontend does not handle that specific file type. To fix this, we moved the filter from compareFullReviewRequest to compareDiff, which is called by both relevant endpoints.

Tests

• create a review request when the only file changes are placeholder files which are hidden
• FE should block creation of review request
• created a review request when there are file changes other than placeholder files, but is subsequently removed, leaving only placeholder file changes in the review request
• FE shows 0 file changes but does not block closing and merging for review request
• server should not throw any error

[kishore03109]

@alexanderleegs @caando sorry may I get your help to dot down the edge cases that yall tested already?

[caando]

@kishore03109 we tested this 2 cases:

Test: attempt to create a review request when the only file changes are placeholder files which are hidden
Response: FE blocks creation of review request

Test: created a review request when there are file changes other than placeholder files, but is subsequently removed, leaving only placeholder file changes in the review request
Response: FE shows 0 file changes but does not block closing and merging for review request

Neither cases led to a server error