If we use scalar clone <url> for a repository that does not include a master branch, then we get this error:

Error: Failed to complete regular clone: error: pathspec 'master' did not match any file(s) known to git

We should respect the default branch name discovered during clone.

(This affects the non-GVFS Protocol case as well.)

cc: @dscho for advice for the proper way to detect the default branch name after a clone.