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revlog: move the `trypending` logic from the `changelog` to the `revlog`

Authored by marmoute.

Description

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