inotify: make mask a class variable since it's instance-independant

inotify: RepoWatcher.updatestatus: document & use meaningful parameter names

inotify: server.walk(): use yield instead of for iterate on subdir when the directory is found, not once all the directories are found, since yield order doesn't matter

context: replace pseudo-set by real set

highlight: updated changed test output It was changed in 783c8c78db86, but I forgot to update it.

byterange: fix import error We must import the module as email.Utils on case-sensitive filesystems.