hgweb: introduction a filerevnav subclass It'll be use to implement the file specific behavior.

hgweb: simplify addition of "(0) navigation entry"

hgweb: simplify the handling of empty repo This abstraction have two advantages: - If the revlog is empty, None of the code bellow is relevant, early returns seems a win. - Abtraction of the 'emptiness' check will help later when we stop relying on nodefunc. A bonus, with filtering, a non-empty revlog may not have '0' revision accessible. It'll be easier to handle with the emptiness test in a dedicated function

hgweb: move hex creation into an object method This is clearer and allow later overwrite.

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)

hgweb: `limit` argument is actually `latestonly` renames and enforce The `limit` argument of several generator have only two possible values in practice: 0 and 1. We rename this parameter to `latestonly` and simplify it's handling. The simplification allows us to save fetching of data that we are sure to not consume. Having a function minimal function perimeter will helps future refactoring.