RepoCommand: Move bare/regular superproject writing to their own classes

RepoCommand: Move bare/regular superproject writing to their own classes

RepoCommand parses the manifest to get a list of projects, clears up
conflicts and then writes to the superproject. The first steps are
common but the writing is completely different for bare or "regular"
(with working dir) repository.

Split writing to bare and regular repos into its own classes. This
simplifies RepoCommand class and makes clearer what happens on each side
(e.g. many options apply only to bare repos).

Change-Id: I256e15729bd53ee15fc56de88bce86a2edb2417a