dulwich: fix output of describe(#4)

[karajan1001]

fix: #4 and iterative/dvc#6051
In dvc we get a random toggling branch name in exp show. It is because
when we iter branches the dulwich.refs.keys gives an unordered Set
of refs. Here we reordered it, and makes current active branch always be
in the first order.

1. Add order to the result of dulwich.refs.keys.
2. Make active branch always in the first place.
3. Add a new test to it.

[karajan1001]

Here the best solution is to order it in dulwich by the time the branch was modified. But I don't know if they wish to modify the return with List instead of Set.

There was an old discussion related to it, but here the suggestion on reflog only gives the time of the last commit, in our case because both of the branch shares the same commit, they would have the same time in reflogs. what we need here is the branch modification time of files in .git/refs/heads
jelmer/dulwich#392

[pmrowla]

Also, this new check for whether or not HEAD is a branch and matches rev when base=refs/heads should maybe be added at the top level Git.describe() instead of at the individual backend level (since the order won't be guaranteed by any backend, but we want to at least be consistent for in this particular case regardless of what backend we are using)

So at the top scmrepo.git.Git.describe() level we would do this specific check (using the generic self.get_ref() calls) and then fall back to returning self._describe() (where _describe = _backend_func("describe"))