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.
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3