Currently, the templating layer tends to treat each namespace
as a one-off, with explicit usage of {bookmarks}, {tags}, {branch},
etc instead of using {namespaces}. It would be really useful if
we could iterate over namespaces and operate on them generically.
However, some consumers may wish to differentiate namespaces by
whether they are built-in to core Mercurial or provided by extensions.
Expected use cases include ignoring non-built-in namespaces or
emitting a generic label for non-built-in namespaces.
This commit introduces an attribute on namespace instances
that says whether the namespace is "built-in" and then exposes
this to the templating layer.
As part of this, we implement a reusable extension for defining
custom names on each changeset for testing. A second consumer
will be introduced in a subsequent commit.
