Enhancement: Add config option to not push branches that do not begin with branch/ prefix

[nathanielcook]

When creating a branch one might forget to add the branches/ prefix. When such a branch is pushed, that branch becomes a Mercurial bookmark, which is a bit of a pain to remove when working with git-remote-hg. (For some discussion on this, see here.) Also we then have to create a new branch at that point, which creates some duplication that could have been avoided had we been alerted somehow of our mistake. It would be really great if developers who do not use hg bookmarks could somehow block creating them inadvertently during push.

[nathanielcook]

For those that want a quick hack, this seems to work for me so far. Put a die statement at line 1144, in do_export.

...
elif ref.startswith('refs/heads/'):
    # bomb completely if there are commits that won't end up in an hg named branch
    die('did you forget to add the branches prefix?')
    ....

@fingolfin Are there any negative side effects to this hack that you know of?