Use default commit sort order instead of TIME

Different versions of libgit2 act differently. Using TIME results in
some commits (like a revert) being listed correctly, but the rest being
listed in reverse order. Leaving it at the default works for
libgit2-0.26.3

src/pylorax/api/recipes.py

@@ -714,7 +714,6 @@ def list_commits(repo, branch, filename, limit=0):
     :raises: Can raise errors from Ggit
     """
     revwalk = Git.RevisionWalker.new(repo)
-    revwalk.set_sort_mode(Git.SortMode.TIME)
     branch_ref = "refs/heads/%s" % branch
     revwalk.push_ref(branch_ref)