branchcache: gather newly closed head in a dedicated set
This is part of a series to more clearly split the update in two step. This
will allow us to introduce a fast path during update in a future changeset.
skip ignored directories if -i or --all not specified
$ hg init t
$ cd t
$ echo 'ignored' > .hgignore
$ hg ci -qA -m init -d'2 0'
$ mkdir ignored
$ ls
ignored
$ hg purge -v --no-confirm
$ ls
ignored