Fix handling of annotated tags when Gitaly is not in use
Attempting to view an annotated tag in the TreeController would result
in NoMethodError: undefined method 'tree' when Rugged was in use.
Blob#find_by_rugged assumes that the ref is a true. Using
the commit ID ensures that the right ref is being used.
Note that in 11.1, Blob#find no longer uses Rugged, so this is only
a bug in 11.0.
Closes gitlab-org/gitlab-ce#47797
CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6423
Edited by Stan Hu