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.