cache cloned git repositories #48
Description
Running vndr with a vendor.conf which contains some large repositories can take quite some time to clone them all every time. This is particularly apparent when iterating adding transitive dependencies of a new package (or today when github is glitchy meaning at least one clone is bound to fail).
It'd be great if vndr could keep a cache of the repositories it has cloned and try to update them rather than reclone every time.
My preference would be to keep the cache in $XDG_CACHE_HOME/vndr (where $XDG_CACHE_HOME defaults to $HOME/.cache) so my existing backup rules will ignore the cache.