libify git-ls-files directory traversal

libify git-ls-files directory traversal

This moves the core directory traversal and filename exclusion logic
into the general git library, making it available for other users
directly.

If we ever want to do "git commit" or "git add" as a built-in (and we
do), we want to be able to handle most of git-ls-files as a library.

NOTE! Not all of git-ls-files is libified by this.  The index matching
and pathspec prefix calculation is still in ls-files.c, but this is a
big part of it.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>