gitignore: lazily find dtype

gitignore: lazily find dtype

When we process "foo/" entries in gitignore files on a system
that does not have d_type member in "struct dirent", the earlier
implementation ran lstat(2) separately when matching with
entries that came from the command line, in-tree .gitignore
files, and $GIT_DIR/info/excludes file.

This optimizes it by delaying the lstat(2) call until it becomes
absolutely necessary.

The initial idea for this change was by Jeff King, but I
optimized it further to pass pointers to around.

Signed-off-by: Junio C Hamano <gitster@pobox.com>