wildmatch: adjust "**" behavior

wildmatch: adjust "**" behavior

Standard wildmatch() sees consecutive asterisks as "*" that can also
match slashes. But that may be hard to explain to users as
"abc/**/def" can match "abcdef", "abcxyzdef", "abc/def", "abc/x/def",
"abc/x/y/def"...

This patch changes wildmatch so that users can do

- "**/def" -> all paths ending with file/directory 'def'
- "abc/**" - equivalent to "/abc/"
- "abc/**/def" -> "abc/x/def", "abc/x/y/def"...
- otherwise consider the pattern malformed if "**" is found

Basically the magic of "**" only remains if it's wrapped around by
slashes.

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