remote: failing test for rename

When there is a reference in the target namespace, we should overwrite
it. Instead it gets a different name under the current code.
remote: tighten up reference renaming

Tighten up which references we consider for renaming so we don't try to
rename unrelated ones and end up with unexplained references.

If there is a reference on the target namespace, git overwrites it, so
let's do the same.
Merge pull request #2404 from libgit2/cmn/remote-rename-fixes

Remote rename fixes