need a way to define final redirect url

my use case is to display jupyter classic or lab inside an iframe (surrounding app is only about table of contents)

in that context, the final redirection url is not dependent on any of the other arguments passed to `git-pull` but is computed by the app; I did try to take advantage of the existing logic for computing that final url, but had a hard time trying to figure what that logic was about, and in the end could not get that to work for me

so I would argue that this should be configurable as-is by the caller, through e.g a `redirectUrl` http arg.