lock-checker: new contrib extension based on work done by Mads

This makes it possible to do lock validation as part of a normal test
run. I didn't attempt any wlock validation because that's a bit more
subtle to detect properly. Thanks to the initial patch from Mads for
the idea.
filter: `updatebranchcache` during `addchangegroup` instead of after lock

The forced recomputation of the branch cache was introduced by `ee317dbfb9d0`.
Back there, `addchangegroup` did not handle any lock logic.

Later `ee1ed6afac21` introduced lock logic to `addchangegroup`. Its description
does not explain why the `updatebranchcache` call is made outside locking. I
believe that the lock was released there because it fit well with the transaction
release already in the code.

Finally `926a06f7a353` moved all "unlocked" code of `addchangegroup` to an
`repo._afterlock` callback.

I do not think that the call to `updatebranchcache()` requires to be done
outside locking. That may even be a bad idea to do so. Bringing this call back
in the `addchangegroup` function makes the flow simpler and eases the following
up changelog level filtering business.
clfilter: introduce a `hassecret` function

We can only use copy clone if the cloned repo do not have any secret changeset.
The current method for that is to run the "secret()" revset on the remote repo.
But with proper filtering of hidden or unserved revision by the remote this
revset won't return any revision even if some exist remotely. This changeset
adds an explicit function to know if a repo have any secret revision or not.

The other option would be to disable filtering for the query but I prefer the
approach above, lighter both regarding code and performance.
clfilter: make the revlog class responsible of all its iteration

This prepares changelog level filtering. We need the algorithms used in revlog to
work on a subset of revisions.  To achieve this, the use of explicit range of
revision is banned. `range` and `xrange` calls are replaced by a `revlog.irevs`
method. Filtered super class can then overwrite the `irevs` method to filter out
revision.
clfilter: handle non contiguous iteration in `revlov.headrevs`

This prepares changelog level filtering.  We can't assume that any revision can
be heads because filtered revisions need to be excluded.

New algorithm:
- All revisions now start as "non heads",
- every revision we iterate over is made candidate head,
- parents of iterated revisions are definitely not head.

Filtered revisions are never iterated over and never considered as candidate
head.
clfilter: split `revlog.headrevs` C call from python code

Make the pure python implementation of headrevs available to derived classes. It
is important because filtering logic applied by `revlog` derived class won't
have effect on `index`. We want to be able to bypass this C call to implement
our own.
clfilter: remove usage of `range` in favor of iteration over changelog

If we want to apply filtering at changelog level, we need to iterate over it.
See previous changeset description for details.
clfilter: remove any explicit revision number from default cmdutil range

Revision "0" and "-1" may be filtered, we can't use them in any default
revrange.
clfilter: introduce `filteredrevs` attribute on changelog

This changeset allows changelog object to be "filtered". You can assign a set of
revision numbers to the `changelog.filteredrevs` attributes. The changelog will
then pretends these revision does not exists in this repo.

A few methods need to be altered to achieve this behavior:

- tip
- __iter_
- irevs
- hasnode
- headrevs

For consistency and to help debugging, the following methods are altered too.
Tests tend to show it's not necessary to alter them but have them raise proper
exception helps to detect bad acces to filtered revisions.

- rev
- node
- linkrev
- parentrevs
- flags

The following methods would also need alteration for consistency purpose but
this is non-trivial and not done yet.

- nodemap
- strip

The C version of headrevs is not run if there is any revision to filter. It'll
need a proper rewrite later to restore performance.