exclude: fix a bug in prefix compare optimization

exclude: fix a bug in prefix compare optimization

When "namelen" becomes zero at this stage, we have matched the fixed
part, but whether it actually matches the pattern still depends on the
pattern in "exclude". As demonstrated in t3001, path "three/a.3"
exists and it matches the "three/a.3" part in pattern "three/a.3[abc]",
but that does not mean a true match.

Don't be too optimistic and let fnmatch() do the job.

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