Don't copy .git directory when making a local charm.

If a charm is under git version control and the developer has bzr-git installed then amulet fails with a bzr error when amulet.deployer tries to initialize a new bzr branch. This is because bzr now recognizes the branch (format is 'git') and refuses to 'bzr init' it, returning an error.

Since juju_deployer really expects bzr branches, the easiest fix is to copy over the tree but not the .git control directory. Then amulet.deployer can initialize it as bzr and everyone is happy.

The is_branch method is a bit of overkill left over from an attempt that looked for .bzr and .git but it is nice to leave in.