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

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>