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 repoview class which will contains
the hidden revs which should be exception.
This will allow to set different exceptions for different repoview objects
backed by the same unfiltered repo.
This patch also adds API to add revs to the attribute set and get them.
Thanks to Jun for suggesting the use of repoview class instead of localrepo.
nit: do not need a space around """