At present, .gitignore patterns not starting with '/' are classified
as "ignored names" (opposing to "ignored paths") and not used for
filtering directories. But, according to the spec [1], the situation
is a bit different: all patterns apply to directories (and those
ending with '/' apply to directories only). Besides that, there two
kinds of patterns: those that match only w.r.t the directory where
defining .gitignore is located (they must contain a '/' in the
beginning or in the middle), which we call "absolute", and those that
also match in all subdirectories under the directory where defining
.gitignore is located (they must not contain '/' or contain only
trailing '/'), which we call "relative".
This commit implements handling of both "absolute" and "relative"
.gitignore patterns according to the spec. "Absolute" patterns are
handled mostly like `ignored_paths` were handled in the previous
implementation. "Relative" patterns are collected into a distinct set
containing `(defining_gitignore_dir, pattern)` tuples. For each
traversed `root_folder_io`, all applicable "relative" patterns are
expanded into a set of plain paths, which are then used for filtering
`folder_io`s.
While at it, also fix some minor issues. Explicitly ignore negative
and wildcard patterns, since we don't handle them correctly
anyway. Also, use '/' as a path separator instead of `os.path.sep`
when dealing with .gitignore, since the spec explicitly says that '/'
must be used on all platforms.
[1] https://git-scm.com/docs/gitignore
