git-read-tree.txt: correct sparse-checkout and skip-worktree description

git-read-tree.txt: correct sparse-checkout and skip-worktree description

The description of .git/info/sparse-checkout and
skip-worktree is exactly the opposite of what is true, which is:

If a file matches a pattern in sparse-checkout, then (it is to be
checked out and therefore) skip-worktree is unset for that file;
otherwise, it is set (so that it is not checked out).

Currently, the opposite is documented, and (consistently) read-tree's
behavior with respect to bit flips is descibed incorrectly.

Fix it.

In hindsight, it would have been much better to have a "sparse-ignore"
or "sparse-skip" file so that an empty file would mean a full checkout,
and the file logic would be analogous to that of .gitignore, excludes
and skip-worktree.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>