branchcache: move the processing of the new data in a dedicated method
In a future changeset, this will allow the V3 of the branch cache to use a fast
path when possible.
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