We have repoview.pinnedrevs() which take a repository object and returns the
hidden revisions which should not be filtered. To add more hidden revisions to
prevent them from getting obsoleted, we need to wrap the pinnedrevs function.
This patch adds repo._pinnedrevs which can be updated with the revisions which
needs to be prevented from being filtered. After this patch
repoview.pinnedrevs() updates repo._pinnedrevs and returns it.
The functionality to add commits to _pinnedrevs without wrapping the function
will be very helpful is adding support for accessing hidden commits.