topic-ext: restrict the format of topic names
This patch restrict the format of topic names to make sure they consist only of
alphanumeric, '_' and '-' characters.
serverminitopic: wrap context.changectx.branch instead of context.changectx
Recently some changes in core resulted in this wrapping of class leading to an
infinite recursion. Let's wrap the particular function instead of wrapping the
whole command.
This fixes test-minitopic.t which was breaking.