clone: support creating mirrors

[efiop]

We've been discussing clone caching before multiple times (e.g. in #41) and it always seemed pretty involved with unshallowing and other potential conflicts.

Seems like we could introduce a mirror concept that would allow you to establish your local cached "mirror" with multiple repos cached by token(url)(or smth like that) and that would use git clone's --mirror flag.

E.g. in dvc we would create our mirror (say in platformdirs.site_cache_dir to avoid strange nfs/cifs/etc problems) and would use it to clone stuff we need:

from scmrepo import Mirror

mirror = Mirror(platformdirs.site_cache_dir(...))

repo = mirror.clone("https://github.com/iterative/example-get-started", ...)
...

[johnyaku]

We'd appreciate it if this could be worked out too.

Is there a case for "installing" import dependencies as git submodules? Then we'd only have to git pull each import source and checkout to the appropriate commit. Or maybe this suffers from problems already considered?

[efiop]

@johnyaku I'm not sure I understand what you mean by installing import dependencies as git submodules, could you elaborate?

[efiop]

Will need to take #250 into consideration as well.

[pmrowla]

The PR implements support for --mirror but I'm not sure that we actually want to use it in DVC, since it will potentially include a lot of refs we don't care about (i.e. Git Notes plus any other private/custom ref namespaces that aren't DVC experiments).

In DVC we probably just want --bare and to set up our own fetch refspec that would only include tags/branches (and maybe DVC exps, but after #250 it may be better to only fetch exps lazily when needed).