wildmatch: make /**/ match zero or more directories

"foo/**/bar" matches "foo/x/bar", "foo/x/y/bar"... but not
"foo/bar". We make a special case, when foo/**/ is detected (and
"foo/" part is already matched), try matching "bar" with the rest of
the string.

"Match one or more directories" semantics can be easily achieved using
"foo/*/**/bar".

This also makes "**/foo" match "foo" in addition to "x/foo",
"x/y/foo"..

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

t/t3070-wildmatch.sh

@@ -63,11 +63,17 @@ match 1 1 ']' ']'
 match 0 0 'foo/baz/bar' 'foo*bar'
 match 0 0 'foo/baz/bar' 'foo**bar'
 match 0 1 'foobazbar' 'foo**bar'
+match 1 1 'foo/baz/bar' 'foo/**/bar'
+match 1 0 'foo/baz/bar' 'foo/**/**/bar'
+match 1 0 'foo/b/a/z/bar' 'foo/**/bar'
+match 1 0 'foo/b/a/z/bar' 'foo/**/**/bar'
+match 1 0 'foo/bar' 'foo/**/bar'
+match 1 0 'foo/bar' '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'
 match 1 1 'foo-bar' 'f[^eiu][^eiu][^eiu][^eiu][^eiu]r'
-match 0 0 'foo' '**/foo'
+match 1 0 'foo' '**/foo'
 match 1 x '/foo' '**/foo'
 match 1 0 'bar/baz/foo' '**/foo'
 match 0 0 'bar/baz/foo' '*/foo'