pygit: use anonymous remotes

[pmrowla]

          This is probably another indicator that we should really be using `git_remote_add_anonymous` (since we technically are breaking the config until we actually hit our `finally:` cleanup block)

Originally posted by @pmrowla in #177 (comment)

User reports there are still cases where our temp remotes are getting written to git config, we need to just use explicitly anonymous remotes instead.

requires libgit2/pygit2#1229

[shcheklein]

@pmrowla it seems it's breaking the Studio update (gto depends on the latest scmrepo). The symptom I see, is that we are getting: exp refs like:

2023-10-27 23:08:29,525 DEBUG scmrepo.git.backend.pygit2 fetch_refspecs: ['+refs/exps/72/31aed430fff6728c2057c6a5985a138725c70b/itchy-cham:refs/remotes/None/exps/72/31aed430fff6728c2057c6a5985a138725c70b/itchy-cham']

Mind the None part in the ref name.

It might be breaking some merge logic (when exp already exists locally). I can confirm that.