Fix: repohasfile filter: only add repos that exist in original repoSet

Created by: attfarhan

Fixes a potential invalid memory address or nil pointer dereference error when using the repohasfile filter. If you had a query like repo:test-2 repohasfile:foo bar and you have two repositories on your instance github.com/test/test-1 and github.com/test/test-2, and both repos had a file foo, the new repoSet would include test-2 and test-1, while we only actually want to add test-2, since that the repo: filter was already accounted for before.

Test plan: added a unit test.