appview/models: fix redundant indirection

PullSource already contains RepoAt, we do not have to access the
optional Repo field.

Signed-off-by: oppiliappan <[e-mail]>

appview/models/pull.go

···
 		source.Branch = p.PullSource.Branch
 		source.Sha = p.LatestSha()
 		if p.PullSource.RepoAt != nil {
-			s := p.PullSource.Repo.RepoAt().String()
+			s := p.PullSource.RepoAt.String()
 			source.Repo = &s
 		}
 	}