When searching for merge requests, an additional subquery
is added which by default filters only merge requests which belong
to source or target project user has permission for.
This filter is not needed because more restrictive filter
which checks if user has permission for target project
is used in the query.
So unless a custom projects filter is used by user, it's possible
to skip the default projects filter and speed up the final query.
Related to #40540
d2f1d585e1