This continues the work to make the `View` types be only about the
state of the current view and not about operations in general (which
has been moved out `OpStore` and qOpHeadsStore`).
Mercurial's "phase" concept is important for evolution, and it's also
useful for filtering out uninteresting commits from log
output. Commits are typically marked "public" when they are pushed to
a remote. The CLI prevents public commits from being rewritten. Public
commits cannot be obsolete (even if they have a successor, they won't
be considered obsolete like non-public commits would).
This commits just makes space for tracking the public heads in the
View.
Git refs are important at least for understanding where the remote
branches are. This commit adds support for tracking them in the view
and makes `git::import_refs()` update them.
When merging views (either because of concurrent operations or when
undoing an earlier operation), there can be conflicts between git ref
changes. I ignored that for now and let the later operation win. That
will probably be good enough for a while. It's not hard to detect the
conflicts, but I haven't yet decided how to handle them. I'm leaning
towards representing the conflicting refs in the view just like how we
represent conflicting files in the tree.
