hgweb: pass nodefunc to the revnav object The issue between hgweb and filtering lay in this function. Moving it into the object itself helps to abstract the erroneous bit.

hgweb: move revnavgen into an object For later compatibility with changelog filtering some part of the navigation generation logic will be altered. Those altered part will be different when in the changelog case and in the filelog case. Moving this into an object will allow to use inheritance to override just the part of the logic we need. The aimed logic are for example: - generation of revision 'hex' (different logic for changelog and filelog) - revlog emptyness test - fetching of the first revision of a revlog (may not be 0)