Deal with nil branch

default_branch doesn't guarantee a result. This checks for the nil value
and raises an exception.

Author: ekohl

lib/modulesync/repository.rb

@@ -39,6 +39,9 @@ def default_branch
     def switch_branch(branch)
       unless branch
         branch = default_branch
+        unless branch
+          raise "Unable to detect default branch"
+        end
         puts "Using repository's default branch: #{branch}"
       end
       return if repo.current_branch == branch