namespaces: add nodemap property
This patch adds a node-to-name map property to the namespace. This is necessary
because we cannot simply invert the name-to-node map because we do not assume
the name-to-node map to be unique (for example, consider named branches: many
nodes have one branch name).
The node-to-name is helpful in log commands where we are already iterating over
a set of nodes and want to display some kind of naming information to the user.
% creating repo
% add and commit
C test.py
% change
M test.py