Custom --ignore-file-name

[baryluk]

--ignore-file-name=FOO

When set, instead of inspecting .ignore and ~/.fdignore, only look for file FOO in each directory, and use that in the logic.

This is useful when having multiple traversal policies, or adapting existing ignore files from other project (i.e. backup softwares, borg - .EXCLUDE.BACKUP, kopia - .kopiaignore).

If file is present, but empty (modulo empty lines), ignore all, including subdirs. If present, but non-empty, interpret similar to ignore/gitignore/kopiaignore in general.