postgresql: Ensure a minimum limit for the snapshot branches query
With small limits (< 10), the snapshot branches query can degenerate into using the deduplication index on snapshot_branch (name, target, target_type), and the postgresql planner happily scans several hundred million rows.
So ensure a minimum limit value of 10 before executing the query for
optimal performances when a small branches_count
value is provided
to the snapshot_get_branches
method of the Storage interface.
Related to swh/meta$963
Test Plan
Testing the snapshot_get_branches
method with small branches_count
values is already done in current tests.
Migrated from D5191 (view on Phabricator)