clfilter: introduce a "unserver" filtering mode This mode is for repository used as a server. It filter secret and hidden changeset out. It is put to use in later changeset.

clfilter: add a cache on repo for set of revision to filter for a given set. Recomputing the filtered revisions at every access to changelog is far too expensive. This changeset introduce a cache for this information. This cache is hold by the repository (unfiltered repository) and invalidated when necessary. This cache is not a protected attribute (leading _) because some logic that invalidate it is not held by the local repo itself.

clfilter: add actual repo filtering mechanism We add a `filtered` method on repo. This method return an instance of `repoview` that behaves exactly as the original repository but with a filtered changelog attribute. Filters are identified by a "name". Planned filter are `unserved`, `hidden` and `mutable`. Filtering the repository in place what out of question as it wont not allows multiple thread to share the same repo. It would makes control of the filtering scope harder too. See the `repoview` docstring for details. A mechanism to compute filtered revision is also installed. Some caches will be installed in later commit.

inotify: pacify pestiferous pyflakes precipitously

tests: make test-inotify-issue1208.t disappear

posix: move server side of unix domain sockets out of inotify We also turn the unix domain socket into a class, so that we have a sensible place to hang its logically related attributes and behaviour. We'll shortly want to reuse this in other code.

inotify: don't fall over just because of a dangling symlink Previously, the inotify server failed to start if .hg/inotify.sock was a symlink that pointed to a non-existent path. This behaviour does not seem to make any sense. Now, if we encounter a broken symlink, we unlink it and continue.

test-inotify: test symlink indirection for unix sockets The inotify code performs a delicate dance to work around the 108-byte limit on unix domain socket path names on Linux. This change sets us up to safely refactor that code without breaking it. (It is redundant with part of test-inotify-issue1208.t, but we will shortly make that test go away.)

test-pathencode: compare current pathencoding implementations We already have two implementations of the pathencoding (C and Python) and this test can perfectly well be used to probabilistically test them instead of just wasting CPU cycles and test time.

rebase: use lazy ancestor membership testing For a repository with over 400,000 commits, rebasing one revision near tip, this avoids one walk up the DAG, speeding the operation up by around 0.8 seconds.