stabletailgraph: add test cases challenging the open merge stack
This adds three more complex test cases with situations requiring tricky
state update in the stack-based iteration (arriving soon).
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