On integration tests file collection,
the value of `environment.executable` can also be a symlink
(e.g. in a virtualenv) with a different name than,
but pointing to the same as `sys.executable`
(e.g. .../bin/python3.10 and .../bin/python, respectively).
That causes skipping the collection of `completion/pytest.py`
and `completion/conftest.py` a lot of times, depending on the environment.
(e.g. "60 skipped" before x "23 skipped" after, in a local virtualenv)
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
