This function opens the depot in magit. By default the depot path is
assumed to be `/depot`, but that is made available as a customise
option that users can override.
Change-Id: Iefc127cad515dbae330d7af3aa69a0c53aa02015
Reviewed-on: https://cl.tvl.fyi/c/depot/+/504
Reviewed-by: glittershark <grfn@gws.fyi>