When handling attr matching, simply compare the directory path where the
attribute file resides to the path being matched.  Skip over commonality
to allow us to compare the contents of the attribute file to the remainder
of the path.

This allows us to more easily compare the pattern directly to the path,
instead of trying to guess whether we want to compare the path's basename
or the full path based on whether the match was inside a containing
directory or not.

This also allows us to do fewer translations on the pattern (trying to
re-prefix it.)

When determining whether some file matches an attr pattern, do
not try to truncate the path to pass to fnmatch.  When there is
no containing directory for an item (eg, from a .gitignore in the
root) this will cause us to truncate our path, which means that
we cannot do meaningful comparisons on it and we may have false
positives when trying to determine whether a given file is actually
a file or a folder (as we have lost the path's base information.)

This mangling was to allow fnmatch to compare a directory on disk to
the name of a directory, but it is unnecessary as our fnmatch accepts
FNM_LEADING_DIR.

When ignoring a path "foo/", ensure that this is actually a directory,
and not simply a file named "foo".

Minimizing the number directory and file opens, minimizes the amount of IO thus reducing the overall cost of performing ignore operations.

Closes #2966.