revlog: move the `trypending` logic from the `changelog` to the `revlog` We move the -reading- logic for the pending's '.a' suffixed index within the revlog class. This is motivated by the fact the logic could be simpler and cleaner if directly handled by the revlog docket. Before we can do so, we need to teach the revlog code about reading "pending" changes. To be honest, we already needed some special casing of the `.a` postfix, so this does not adds much complexity. The logic around -writing- the special '00changelog.i.a' remains in the `changelog` class. Note that the revlog-v2 logic no longer use this logic. The only remaining user of the `postfix` argument is the `censor` logic. We could probably also make the revlog full aware of it (most of the code is already implemented in revlog anyway) and get rid of the `postfix` argument and logic. However this is an adventure for another time. Since we have more information, we add more, paranoid, Programming error in case we detect such "pending reader" trying to do a read (which does not happens anyways). Differential Revision: https://phab.mercurial-scm.org/D10630