git-read-tree.txt: update sparse checkout examples

The negation example uses '*' to match everything. This used to work
before 9037026 (unpack-trees: fix sparse checkout's "unable to match
directories") because back then, the list of paths is used to match
sparse patterns, so with the patterns

    *
    !subdir/

subdir/ always matches any path that start with subdir/ and "*" has no
chance to get tested. The result is subdir is excluded.

After the said commit, a tree structure is dynamically created and
sparse pattern matching now follows closely how read_directory()
applies .gitignore. This solves one problem, but reveals another one.

With this new strategy, "!subdir/" rule will be only tested once when
"subdir" directory is examined. Entries inside subdir, when examined,
will match "*" and are (correctly) re-added again because any rules
without a slash will match at every directory level. In the end, "*"
can revert every negation rules.

In order to correctly exclude subdir, we must use

    /*
    !subdir

to limit "match all" rule at top level only.

"*" rule has no actual use in sparse checkout and can be confusing to
users. While we can automatically turn "*" to "/*", this violates
.gitignore definition. Instead, discourage "*" in favor of "/*" (in
the second example).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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