filemerge: fix crash when using filesets in [partial-merge-tools]
Without this patch, you'd get `mercurial.error.ProgrammingError: fileset
expression with no context`.
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