check user and repo for redirects when using git via SSH transport (#35416)

fixes #30565 

When using git with a gitea hosted repository, the HTTP-Transport did
honor the user and repository redirects, which are created when renaming
a user or repo and also when transferring ownership of a repo to a
different organization. This is extremely helpful, as repo URLs remain
stable and do not have to be migrated on each client's worktree and
other places, e.g. CI at once.

The SSH transport - which I favor - did not know of these redirections
and I implemented a lookup during the `serv` command.

models/fixtures/user_redirect.yml

@@ -2,3 +2,7 @@
   id: 1
   lower_name: olduser1
   redirect_user_id: 1
+-
+  id: 2
+  lower_name: olduser2
+  redirect_user_id: 2