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>

t/t3070-wildmatch.sh

@@ -46,7 +46,7 @@ match 0 0 foobar 'foo\*bar'
 match 1 1 'f\oo' 'f\\oo'
 match 1 1 ball '*[al]?'
 match 0 0 ten '[ten]'
-match 1 1 ten '**[!te]'
+match 0 1 ten '**[!te]'
 match 0 0 ten '**[!ten]'
 match 1 1 ten 't[a-g]n'
 match 0 0 ten 't[!a-g]n'
@@ -61,7 +61,8 @@ match 1 1 ']' ']'
 
 # Extended slash-matching features
 match 0 0 'foo/baz/bar' 'foo*bar'
-match 1 0 'foo/baz/bar' 'foo**bar'
+match 0 0 'foo/baz/bar' 'foo**bar'
+match 0 1 'foobazbar' 'foo**bar'
 match 0 0 'foo/bar' 'foo?bar'
 match 0 0 'foo/bar' 'foo[/]bar'
 match 0 0 'foo/bar' 'f[^eiu][^eiu][^eiu][^eiu][^eiu]r'