This patch adds two new filternames 'visible-hidden' and 'visible-warnhidden'
which will be used to access hidden commits.
'visible-hidden' will allow access of hidden commits with any warning. This will
be for read only commands.
'visible-warnhidden' will allow acces of hidden commits but will warn for that.
This will be for recoverable write commands.
For unrecoverable write commands, we don't need to have anything new as that's
the current behaviour.
We are using the same computehidden() function as we will be adding the hidden
revisions to pinnedrevs.
The idea to add new filternames is taken from directaccess extension from