Allow building snap from current default branch of repository

[Colin Watson]

Launchpad allows building a snap from any branch in a Git repository, if named explicitly. However, it would be useful to be able to have a way to represent the notion of building from whatever the current default branch is: for instance, I think this would be a clean way to fix https://github.com/canonical-ols/build.snapcraft.io/issues/497.

We generally prefer to pass IGitRef-providing objects around internally rather than explicit repository/path combinations (see e.g. GitRefRemote), and I don't want to unwind that. However, the repository scanner doesn't and probably shouldn't create GitRef rows for HEAD, instead just setting GitRepository.default_branch.

I think the most elegant way to handle this would be as follows:

 * Allow launchpad-buildd to accept a snap build without git_path, turning that into a simple 'git clone' rather than 'git clone -b <git_path>'.
 * Extend GitRepository.getRefByPath so that if path is HEAD then it returns a new flavour of IGitRef which refers to whatever the current default branch is.
 * Adjust SnapBuildBehaviour to omit git_path if one of these "use the default branch" GitRef objects is passed.

[Colin Watson (cjwatson)]

Fixed in launchpad-buildd 143 (but there's still a branch to land on the Launchpad side).

[Launchpad QA Bot (lpqabot)]

Fixed in stable r18384 <http://bazaar.launchpad.net/~launchpad-pqm/launchpad/stable/revision/18384>.