Fix bug where transferring a project would result in stale commit links
Transferring a project to another namespace updates the project's updated_at
field, but since the cache key did not depend on the object, the page fragments
were not invalidated. This resulted in stale links to the commits. Changing
the cache key to use the project pathname solves this issue.
Closes gitlab-org/omnibus-gitlab#843
See merge request !1497
