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>