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revlog: introduce a mandatory `_writing` context to update revlog content

Before this change, various revlog methods where managing the opening and
closing of the revlog files manually and passing the file descriptor alors the
call path. To simplify the tracking of the write operation by a future docket,
we need something more organised. As a result, we introduce a `revlog._writing`
context manager that will wrap each revlog update operation. The file
descriptor are kept in the existing `revlog._writinghandles` parameter that
was already used by the `addgroup` logic.

All this change is internal to the revlog only, the "public" interface is not
affected. The `addrevision` and `addgroup` logic are still responsible for
setup up this context. However this new context give us multiple benefits:

* all writer use a same, unified, logic,
* this context is programmatically enforced,
* each write "session" as a clearly identified start and end.

The post-pull sidedata update logic is still doing writing by end and will be
adjusted in a later changesets.

This change affect the concurrency checker test, because register the state of
the file in the transaction sooner in `addrevision` (about as early as what
`addgroup` would do), so the abort is rollbacking the other commit. I don't want
to weaken the current main logic.

Differential Revision: https://phab.mercurial-scm.org/D10605

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