Redirect to new repository owner (#21398)

Fixes #17655

If you rename `user1` to `user2` and visit `/user1` you get redirected
to `/user2`. But if you visit `/user1/repo` you just get a 404 error.
With this PR the user is redirected to `/user2/repo`.

modules/context/repo.go

@@ -451,11 +451,20 @@ func RepoAssignment(ctx *Context) (cancel context.CancelFunc) {
 		owner, err = user_model.GetUserByName(ctx, userName)
 		if err != nil {
 			if user_model.IsErrUserNotExist(err) {
+				// go-get does not support redirects
+				// https://github.com/golang/go/issues/19760
 				if ctx.FormString("go-get") == "1" {
 					EarlyResponseForGoGetMeta(ctx)
 					return
 				}
-				ctx.NotFound("GetUserByName", nil)
+
+				if redirectUserID, err := user_model.LookupUserRedirect(userName); err == nil {
+					RedirectToUser(ctx, userName, redirectUserID)
+				} else if user_model.IsErrUserRedirectNotExist(err) {
+					ctx.NotFound("GetUserByName", nil)
+				} else {
+					ctx.ServerError("LookupUserRedirect", err)
+				}
 			} else {
 				ctx.ServerError("GetUserByName", err)
 			}