Currently we don't have a defined way in core to make some hidden revisions
visible in filtered repo. Extensions to achieve the purpose of unhiding some
hidden commits, wrap repoview.pinnedrevs() function.
To make the above task simple and have well defined API, this patch adds a new
attribute '_visibilityexceptions' to localrepository class which will contains
the hidden revs which should be exception.
This patch also adds API to add revs to the attribute set and get them.