The gitignore's pattern format specifies that "Trailing spaces
are ignored unless they are quoted with backslash ("\")". We do
not honor this currently and will treat a pattern "foo\ " as if
it was "foo\" only and a pattern "foo\ \ " as "foo\ \".

Fix our code to handle those special cases and add tests to avoid
regressions.

Unlike ignore files, gitattribute files can have flexible whitespace at
the beginning of the line.  Ensure that by adding new ignore rules that
we have not impeded correct parsing of attribute files.

The function `git_ignore_path_is_ignored` is there to test the
ignore status of paths that need not necessarily exist inside of
a repository. This has the implication that for a given path, we
cannot always decide whether it references a directory or a file,
and we need to distinguish those cases because ignore rules may
treat those differently. E.g. given the following gitignore file:

    *
    !/**/

we'd only want to unignore directories, while keeping files
ignored. But still, calling `git_ignore_path_is_ignored("dir/")`
will say that this directory is ignored because it treats "dir/"
as a file path.

As said, the `is_ignored` function cannot always decide whether
the given path is a file or directory, and thus it may produce
wrong results in some cases. While this is unfixable in the
general case, we can do better when we are being passed a path
name with a trailing path separator (e.g. "dir/") and always
treat them as directories.

When comparing whether a path matches a directory rule, we pass the
both the path and directory name to `fnmatch` with
`GIT_ATTR_FNMATCH_DIRECTORY` being set. `fnmatch` expects the pattern to
contain no trailing directory '/', which is why we try to always strip
patterns of trailing slashes. We do not handle that case correctly
though when the pattern itself has trailing spaces, causing the match to
fail.

Fix the issue by stripping trailing spaces and tabs for a rule previous
to checking whether the pattern is a directory pattern with a trailing
'/'. This replaces the whitespace-stripping in our ignore file parsing
code, which was stripping whitespaces too late. Add a test to catch
future breakage.

When computing negative ignores, we throw away any rule which does not
undo a previous rule to optimize. But on case insensitive file systems,
we need to keep in mind that a negative ignore can also undo a previous
rule with different case, which we did not yet honor while determining
whether a rule undoes a previous one. So in the following example, we
fail to unignore the "/Case" directory:

    /case
    !/Case

Make both paths checking whether a plain- or wildcard-based rule undo a
previous rule aware of case-insensitivity. This fixes the described
issue.

Ignore rules allow for reverting a previously ignored rule by prefixing
it with an exclamation mark. As such, a negative rule can only override
previously ignored files. While computing all ignore patterns, we try to
use this fact to optimize away some negative rules which do not override
any previous patterns, as they won't change the outcome anyway.

In some cases, though, this optimization causes us to get the actual
ignores wrong for some files. This may happen whenever the pattern
contains a wildcard, as we are unable to reason about whether a pattern
overrides a previous pattern in a sane way. This happens for example in
the case where a gitignore file contains "*.c" and "!src/*.c", where we
wouldn't un-ignore files inside of the "src/" subdirectory.

In this case, the first solution coming to mind may be to just strip the
"src/" prefix and simply compare the basenames. While that would work
here, it would stop working as soon as the basename pattern itself is
different, like for example with "*x.c" and "!src/*.c. As such, we
settle for the easier fix of just not optimizing away rules that contain
a wildcard.