formally supported method to iterate through all commits

[fche]

Repo.iter_items / iter_commits normally limits itself to parents of HEAD.
However, if a GitPython program wants to iterate through all commits in a git repo, it can use a hack like this:

   r = git.Repo('/path')
   for commit in r.iter_commits('--all'):
        pass

expecting the "--all" flag to be passed down to "git rev-list", and thence back to the python code. It would be nice if there were a more natural interface to expose this search, one that could also work with GitDB.