loader-mercurial: Add default branch's tip as alias in snapshot's branches
Related swh/devel/experiments/swh-db-audit#1257 (moved)
Test Plan
make test
Migrated from D511 (view on Phabricator)
Related swh/devel/experiments/swh-db-audit#1257 (moved)
make test
Migrated from D511 (view on Phabricator)