Improve checks for ignore containment

The diff code was using an "ignored_prefix" directory to track if
a parent directory was ignored that contained untracked files
alongside tracked files. Unfortunately, when negative ignore rules
were used for directories inside ignored parents, the wrong rules
were applied to untracked files inside the negatively ignored
child directories.

This commit moves the logic for ignore containment into the workdir
iterator (which is a better place for it), so the ignored-ness of
a directory is contained in the frame stack during traversal.  This
allows a child directory to override with a negative ignore and yet
still restore the ignored state of the parent when we traverse out
of the child.

Along with this, there are some problems with "directory only"
ignore rules on container directories.  Given "a/*" and "!a/b/c/"
(where the second rule is a directory rule but the first rule is
just a generic prefix rule), then the directory only constraint
was having "a/b/c/d/file" match the first rule and not the second.
This was fixed by having ignore directory-only rules test a rule
against the prefix of a file with LEADINGDIR enabled.

Lastly, spot checks for ignores using `git_ignore_path_is_ignored`
were tested from the top directory down to the bottom to deal with
the containment problem, but this is wrong. We have to test bottom
to top so that negative subdirectory rules will be checked before
parent ignore rules.

This does change the behavior of some existing tests, but it seems
only to bring us more in line with core Git, so I think those
changes are acceptable.
Merge pull request #2328 from libgit2/rb/how-broken-can-ignores-be

Improve checks for ignore containment