refdb: adjust the threading tests to what we promise
refdb: adjust the threading tests to what we promise

We say it's going to work if you use a different repository in each
thread. Let's do precisely that in our code instead of hoping re-using
the refdb is going to work.

This test does fail currently, surfacing existing bugs.

stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2